Jim,
If it's got the 'winged bullet then it certainly was made 'post-merger'.

Have you missed an 'x' off there? I ask because a gun in the 10xxx range would seem to be outside of the normal range of numbers (10xxxx would seem more likely)?

Unless of course the P. Webley numbering system was also used for a year or two after the merger, (later P. Webley branded guns did carry a serial numbers from the normal W&S range).

Regards
Webley