J.W. Benson's Alpha-Numeric Serial Numbers.

J.W. Benson "Bank" watch, 11J, 1922
The case has the same numeric serial number as the movement so the date is pretty certain.

Some Benson "Bank" movements and its derivatives after c 1901 have a serial number prefixed with a letter.  About 12% of the 59 Benson "Bank" watches I have seen have this numbering system but the percentage would be somewhat higher if only watches after 1900 were considered.

Serial number L4381 on the top plate of the 1922 movement shown above.

Some have speculated that this is a date code but I have been unable to find a good correlation.

Things get more confusing if you take off the dial because the serial number prefix on the face plate is always different to that on the top plate.

Serial Number C4381 on the face plate of the 1922 movement shown above.

If the Letter was a date code it would imply that the face plate was made 9 years before the top plate or before the watch was finished which appears unlikely.

Unusually for Benson those movements with an Alphabetic prefix to the serial number often have the same serial number on the case as on the movement but without the prefix which makes the dating certain (unless the watches were re-cased by the maker).

The silver case, hallmarked for J.W. Benson 1922 serial No 4381.

So together with the lack of correlation between the letter and dates it appears the letter represented something else.

But I have no idea what.

The watch from the front.