The penny finally dropped as I took it apart yesterday and made notes, as I do these days. I duly noted down the Wadsworth case serial number xxx9394 and then removed the bezel. By chance, the scratches were lit and oriented such that it looked like VIIII III VIIII IIII - and in Roman numerals that's the last 4 digits of the above serial number! The inscriber hadn't closed the bottom of the V's so, from other angles, it wasn't at all obvious what the scratches represented.
One slightly interesting thing about these particular numerals is that the inscriber didn't use the "subtractive" method, ie IX for 9 and of course, IV for 4 - the which has often been discussed ref. dial numbers. I guess that a V is easier to scribe than a X, and an I is easier than a V.
Ed Ueberall (The Escapement) agreed:
Many of the watch case companies did this in order to keep matching parts together as they traveled around the factory. Often the center section will also have the last four (or more) Arabic digits stamped on the rim, while the bezel had the hand scratched (Roman) numerals. The back cover(s) had the full number stamped inside.
Cary Hurt also mentioned:
You'll also find this on Hamilton wristwatch cases made by both Wadsworth and Fahy's. Though (as Ed says above) this was originally done as an inventory aid during production, it now serves as an easy way to validate the originality of bezel-midsection-back combinations.
This is increasingly important as the rarity of original two-tone watches makes it unfortunately more attractive for mismatched sets to be presented as genuine. It also can help to identify when a gold-filled bezel is mated to a solid gold back.
I've seen it on Wadsworth cases by Elgin, Waltham and Gruen as well.
Terry Hall just added:
If you find an upside down V, that is a Zero.....