Hi Jeremy,
I cannot help you with any info. on Armstrong, but Arthur Howell & Co. used a similar stamp. You did not mention where that mark is on your barrels, but the Arthur Howell stamp that I am referring to is under the right barrel, up near the bottom rib, close to the forward end of the flats, and the mark is styled something like this, with the small "o" underlined.
A.H&Co
I have seen boxlocks that are marked on both the bar and rib with the names C & A Weston, W. Hodgson, Thomas Bland, Henry Monk and Arthur Howell, all with that A.H&Co mark under the barrels, and I understood that those guns were all made by Arthur Howell & Co.
All of those guns were originally proof marked between 1933 and 1939, BTW.
