I have definitely been presented with some mysteries. The complete one is a '76 model. I don't know the build date as the paint is missing. The bare frame is a very early '75 with a 3/75 build date and a 500x serial number. I think the engine in the '76 is from the '75 or even a 3rd bike. The partially complete engine (missing one head, but not stuck) has a s/n close to the 76 frame. The engine in the 76 had had the serial number ground off. All that is left is a "SC02E -" above where the s/n should be. The front wheel on the '76 is garbage. The rim has big chunks flaked off. The back seems ok and the wheels on the other frame are fine, but need to be respoked. Both seats I got are rusted to nothing and worthless. There are some valuable parts, but making them into a single rider will be a lot of work.
It is too bad about the 4-1 header. Note the 02 sensor at the back. The original owner made some interesting mods. Too bad it wasn't stored better.