Are you saying this model could have been built in the late 60's? I was thinking 70's, but who really knows! That's why I'd like to get a hold of the original owner- now I wish I asked him. Today I weighed the model with the YS .45 and pipe, and threw all the radio gear on top of the wing to get an idea of the all up flying weight. Rough estimate- comes out to about 5.5 lbs all up. The .45 is actually big enough to fit right on the rails with no mods- it may even use the existing holes! I've pretty much ruled out the .91 from what you guys are saying, so now it's down to the .45, or a 70's vintage OS black head max. Can't decide!