This is a Hp .61 I recently acquired. It's a pre-'72 with the 4-bolt head. I asked in a previous post about the hole drilled in the exhaust flange. It seems to have been added by a previous owner. The real question I have is the backplate. Every photograph of HP 4-bolts show the standard 4-bolt backplate we're normally accustomed to. Whats up with this one? The engine appears to have been test run, at most, and if it helps, is an early production run, with a serial number in the 50's. I was going to use this on a Phoenix I I'm currently scratch-building after I acquire the original clip-on muffler. Any info would be appreciated....Mark