It's a little hard to make out in the photo, but I have found a Parker Vacumatic with an extra row in the barrel imprint area. It reminds me of the different font used for pricing on the older Sheaffer Balances but obviously this is a Parker and not a Sheaffer. (Those imprints never had letters in them anyway.)
The extra imprint says, "2S1" Does anyone know what it means? It's very clearly an S and not a 5 in hand. It's clear enough to be read without magnification too.
For more information about the pen itself, it is a Parker Vacumatic Major in [Laminated] Azure Pearl with a plastic plunger system and a date code of 5 (so 1945). It has a chevron band and a blue diamond clip. Nothing about this pen seems out of place with the exception of the extra imprint.
It is, as they say, a mystery. Please help me solve the case.