At work don't have engine to look at, but from pics and info, I am certain it is high compression. Fin arrangement over cam cover etc.
If so, then this engine was not desigined to run on 15% nitro, right? Would explain a lot. Just courious, if I had the original instructions for this engine what would the MFG reccomend as a fuel. Have some 5% Wildcat, will try it on this blend tonight and let all know the result.
Don