That looks more like a shell detonation than a clean overpen. You can see scorch marks on the edge of the hole. It was probably from a 20mm.
I've never seen a clean overpen in this game, in that it hits somewhere on one side, punches straight through and out the other.
The damage you sustained in your 51 was likely from an auto-ack though, which I believe are quad 20's? I've honestly forgotten. I've also had a similar hole in the same area in my Spitfire Mk I during a Battle of Britain FSO a few years ago. Myself and another CJ were engaged with 5 110's, and one of them managed to score a hit on me in that fight in the same area on my Spitfire. Left a similar hole as well.
Of all the times I've used the 37mm, I've not once seen a fighter survive a hit to that area from it. And if I recall correctly, the 37mm auto-ack guns are
exactly the same as the manned version with the exception of Rate of Fire. So as I mentioned before, you likely took a hit from an auto-ack 20.
