First, congrats on the successful attack! Early war or not, getting in and out of a convoy with some kills AND your boat is a worthy achievement!
Second, those S boats are out there. I was sailing my XXI towards the Orkneys when a boat opened up on me. At first I thought it was a patrol boat because it was so far away, but it was actually an S-class sub. So I got close underwater and torpedoed it to the bottom. Good times- I think they're worth a good haul of renown IIRC from the other time I bagged one (surface gun duel with my IXB back in 1941).