Thanks Chris and vintagewebleyman for the serial number information. My serial number list now stands as follows:

Single clip model: 27

Double clip model:
30,
118
177
281
219
253
441
469
547
649

If anyone can provide any additional authenticated numbers I would be very grateful.

Undoubtedly the single-clip model is the rarest of the Webley Mark 1 pistols, and the double clip version comes a close second.