Yes, that's interesting indeed. That's pretty much the final proof that the previous owner was mistaken about the RV-4 engine mount. At this point, I'm sure he just didn't recall the details, and was actually referring to the lower firewall weldments. I looked at those yesterday, and they're clearly newer than everything else, with a different paint finish, so the mystery is solved.
Appreciate the replies!
RV-8, SN-80587, built, flown, sold.
RV-3B, SN-10751, rotary engine, built, flown, sold
RV-8, SN-82470, built, flown, sold.
RV-3B, SN-11351, purchased (yes it's cheating)
A&P - 2018 dues paid