.31 Jap certainly isn't a common usage, but you can google and find some references to it.
Several people at my hunting camp when I was a teenager called it that.
It was also used in print, here, bottom of page 57 in the Lowes Powder Horn ad.
Do yourself a favor and don't look at the prices in the ads. You'll cry.
Oh, and Jonny?
There's also an ad in there for .42 Russian Berdan cartridges, Tula arsenal loads, in original string-wrapped six-count packets for $1 each...
Last one of those I saw at a show was $75.