GMCS -
The authoritative source for Manton serial numbers and dates are two books, written by W. Keith Neal and D.H.L. Back. They are: "The Mantons: Gunmakers," 1966, Walker and Company, Library of Congress No. 67-12527, and: "The Manton Supplement," 1978, The Scolar Press, Ltd., ISBN 0 900193 57 3.
Both books have been out of print for years. You may be able to find them through Inter-library Loan, however. They list all the known Manton guns (meaning known to the authors) at the time of publication, with a lot of other information, such as name of manufacturer, date of manufacture, serial number, gauge, style (single barrel, side by side, over-under), etc.
There were a number of legitimate "Manton" family companies, in addition to the spurious Mantons already mentioned, including one in Calcutta, India, believe it or not.
Hope this helps.
Bill