Every 4 cylinder 10 series I ever worked on had 3. Just did a 7600 the other day that had 3. I've seen some older 8000s/8600s with only 2 though. Possibly a swap was made in the past?
Regardless, if you're going to re-use the drum, have it chucked up in a lathe and dig in there a little to see what comes out. I would think one should be able to at least see where the ring gap was.
Worn support horns are caused by improper adjustment of the locating screws on the LH side of the housing. Make sure it is properly set as per the book.