Colts
That is a very nice pistol. I don't know if you have invested $100 in a Colt letter, but here is what I can tell you about your pistol.
It is difficult to give an exact date for the British contract Colts due to Colt's habit of not assembling and shipping pistols in serial number order. Also, their allocation of serial numbers to the "W" series of .455" pistols is confusing as described in my previous post. For example, the RFC/RAF pistols range from W91100 to W110969, approximately 16,500 pistols, yet only 10,000 were manufactured.
Colt records only show the lowest and highest serial in each shipment so one cannot say when a particular pistol was actually shipped.
Your pistol, W91690, could have been in any of the following shipments:
30th March 1917, 300 guns within S/Nos W91100-W95000, shipped to the London Armoury Company, Contract 94/P/1118.
6th April 1917, 200 guns within S/Nos W91600-W97000 shipped to the London Armoury Company, Contract 94/P/1118.
2nd October 1917, 300 guns within S/Nos 91500-W100000 and
2nd November 1917, 200 guns within S/Nos W91600-W100100, both to the London Armoury Company under an Air Service order.
21st February 1918, 100 guns within S/Nos W91100-W100500 shipped to Sir Connop Guthrie, English Gov't, New York, Contract US1139-MM67.
Apart from the serial number range, we know it was shipped before April 1918 from the proof marks. The pennants on the the crossed pennants proof mark of your pistol have split tails, indicating proof at RSAF Enfield. After that date they were inspected and proofed by a British inspector, G.W.R.Steadman, at Colts and the proof mark used then had rounded pennants.
I know, I should get out more...
Regards
TonyE