W Marson was a new name to me. My supposition would be that a firm like MacNaughton would have used one of the better known barrel makers. This is what IGC have on him:
Name William Marson
Address1 76 Great Hampton Row
City/Town Birmingham
Country United Kingdom
Trade Gun maker
Dates 1871?-1885?
Notes
William Marson was the son of John Marson Snr of 55 Livery Street. He was born in Birmingham in 1838, and was the brother of John Jnr and Samuel. In the 1861 census he was recorded as a 23 year old gun finisher. In the 1871 census he was recorded as a gunmaker and retail brewer at 74 Great Hampton Row. He had married Harriet in about 1860, and they had two sons, William (b.1861) and Albert (b.1864); they also had two daughters, Hannah (b.1866) and Clara (b.1880). He appears to have retired or died in about 1885.
In case you would like the Macnaughton history, here it is, courtesy IGC:
Name James MacNaughton
Other Names James MacNaughton & Sons
Address1 33 George Street
Address2 26 Hanover Street South
Address3 36 Hanover Street
City/Town Edinburgh
State/Region/Province Scotland
Country United Kingdom
Trade Gunmaker
Other Address 44 George Street, Perth.
Dates 1864-1947
Notes
James MacNaughton was born in Edinburgh in 1838. He was apprenticed to John Dickson in 1852 and from 1859 to 1864 worked for him as a journeyman. In 1864 he established his own business at 33 George Street, Edinburgh. On 5 July 1867 he patented a sliding drop-down breech action with three interlocking lumps which he called the "Lock Fast Breechloader" (patent No. 1971).
In 1869 he moved to 26 Hanover Street South. In the 1871 census James was recorded as a "gunner" living at 1 Glengyle Terrace with his wife, Isabella (b.1840 in Edinburgh) and their children, Jane (b.1867), Jessie (b.1869) and David (b. 1870). Also living with them was James' mother, Jane (b.1799 in Edinburgh).
James MacNaughton made Field patent falling block rifles, but on 17 April 1872, together with A R Duncan, he registered patent No. 1137 covered their own falling block action. On 12 July 1879 MacNaughton patented a top lever cocking falling block action, extractors, sear springs, firing pins, a safety catch, a crystal cocking indicator port in the top strap (not fitted after about 1890) and his famous round action (No. 2848) in which the locks were placed on the trigger plate which enabled him to create the first round action gun. Unfortunately, the falling block action infringed Alexander Henry's patent and he was forced to discontinue manufacture, but the trigger-plate action (the "Edinburgh" Patent Long Lever Hammerless Ejector Gun), with its long pierced top lever for increased leverage when cocking the gun, became famous. The first one they made, a bar-in-wood version (but never called by this name and usually referred to as a skeletal action) (serial No. 736), was shown at the Great Edinburgh Exhibition of 1886 and then sold. In 1998 it was put up for auction and was bought back by the firm. John Dickson copied parts of MacNaughton's design to make his "round action" barrel cocking gun. Reportedly, MacNaughton took legal action against him for patent infringement but no court records exist and the Dickson patent remained in force. In the 1881 census the family was recorded living at 32 Dundas Street. James was described as a gunmaker employing 13 hands. Two more boys, Alan (b.1873) and Duncan (b.1876) had been born, as had two more daughters, Mary (b.1878) and Catherine (b.1880). On 23 August 1886 James MacNaughton registered patent No. 10750 for an improved sliding bolt mechanism for rifles with tubular magazines. In about 1894 MacNaughton bought the business of David Crockart at 44 George Street, Perth. He later sold it back either to David C Crockart or to David Bissett Crockart. In any event, the business traded as D B Crockart (see D B Crockart).
On 15 June 1899 James MacNaughton registered patent No. 12464 for his "dual" double rifle on which the ribs, apart from a short rib to hold the rear sight, were replaced by sleeves over the two "separate" barrels at the breech and the muzzle. His idea was to improve accuracy by allowing for the dissipation of heat produced by cordite powder. The idea was not well received because cordite did not produce much more heat than other powders, third and fourth shots were seldom taken with a double rifle and even if they were, the marginal improvement in accuracy was seldom of any advantage. This patent also covered a fore-sight cover and rotating striker block for falling block rifles. On 19 April 1890 James MacNaughton registered patent No. 7759 for an ejector mechanism on a slide in the fore-end. In the 1891 census the family was recorded living at 16 Woodburn Terrace. David, Alan and Duncan were all described as apprentice gunmakers but in the 1901 census David described himself as a law agent and Alan, who described himself as a gunmaker, was lodging in Perth because he was managing the Perth branch of the business. It seems that Duncan was destined to take over the firm. By the mid 1890s top lever cocking actions had been superceded by barrel cocking actions and the MacNaughton round action was a copy of the Dickson patent. On 26 JUne 1901 J MacNaughton registered patent No. 13005 for a rifle backsight.
On 17 May 1905 James MacNaughton died (aged 66) and the firm moved to 36 Hanover Street. At the same time, the name of the firm was changed to James MacNaughton & Sons, these being Alan and David. In 1947 the Edinburgh business was sold to John Dickson & Son Ltd. In 1995 the name, goodwill and records of James MacNaughton was sold by John Dickson & Son Ltd to Alastair Laing (of Dicksons) and Barry Wilcox (of Dicksons and formerly of Hendry Ramsay & Wilcox) and B E Chaplin; together they traded as Logie Gunmakers. In 1997 Logie Gunmakers acquired John Dickson & Sons and the two firms amalgamated as Dickson & MacNaughton (see John Dickson & Son). The records of the firm date only from 1894. Internet Gun Club has some details of serial numbers and dates of manufacture which we have not published. Please send details of your gun and its serial number by email to archives@internetgunclub.com and we will reply with what information we have. Further information is available from the firm at 21 Frederick Street, Edinburgh EH2 2NE; Tel: 0131 2254218.
Other Info
The firm made a model named "The Marsupial Express" in about 1880 (32 bore). The firm sold cartridges under their own name and under the names "Brown" (post 1927) and Orange (post 1927).
This is his trade label, used from 1886-95:
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Tim
No 
It is most definitely MacNaughton. 
