At this point in time, anything can show up anywhere at anytime if it is a GENERIC call, which are the RGG (Random Generated Group) calls without a "Class=xxx" line, such as:
[RndGroup 1.RndUnit 1]
Type=4
Origin=Japan
Side=2
The Japanese subs are Type=17, and most of the subs in the game are RGG GENERIC calls. Once we know everything works, their dates might be attempted to be controlled again, but we have found that the game seemingly does what it wants with the dates, as the Early Balao and Early Tench issues can attest, as well as the game pulling "names" out of a hat that we didn't know existed... You will notice the playable submarines will pop-in with names that are not active in their "CFG" file, yet the game uses them. This is a very frustrating aspect of modding the game...
