I think it is in the flotilla.cfg file in data/config or something similar. There it assigns vessel types, #s (u-45, etc), bases, patrol grid orders, etc. I am not sure about specific equipment.
For each Flotilla there are sections that belong to each time period and each section assigns the information above. For your particular flotilla, look at when the base become active and change the base or dates to the one you want. You may want to edit the patrol grids also.
To add a snorkel to a VIIb you might have to find the unit's config file, probably under /config or /submarine, I dont know.
You can look at flotilla mods that have a readme for assistance also. Hope this helps.
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