At conclusion of Patrol 6, I was offered a new command - the second time this has happened this career. I figured I'd accept and see what I got. It was a Tuna class. I had docked on the 29th, so I went back to an earlier save and this time sailed around until I could dock on the 30th. Same result - I ended up with a Tuna class, though it was a different sub.
If I had docked, say, mid-January would I have been offered a Tench? It appeared that any command offered was going to be a Tuna. Anyone have any insights on this? Is the class assigned random?