I was told about this forum yesterday. Great to have a specific source for these old and unique bikes. I have had a 1973 that I pulled out of a junk yard a few years ago. It now runs and rides, but it has a long way to go. Fortunately it does have the adjustable omni-phase balancer recall and the extended sump.
I'll have to look. Mine is at home today, I brought my 1976 GL1000 to work. know there is a serial number on the engine, but I don't remember what it is. Usually the engine and frame serial numbers are different on Japanese bikes, so I am not sure I could tell.