I saw a few 240z years ago with metal "crank back" tops that appeared to be factory. I was a kid but I remember clearly that the sun roofs where metal. Does anyone know if there were a few Z cars with factory roofs?

No, none what so ever.

I have - however - seen pictures of one car with what you describe. And to the eyes of an old BMW owner, it looked to me that it may have been a BMW E21 sunroof mechanism grafted in. The crank, size and other indicators all looked exactly like the sunroof on my '81 323i.

That roof, or other similar crank-type roofs could be installed by a good bodyshop, just the same as how a good shop could replace my car's roof with an unmolested one. It just takes time and money.

Wow, if that's the case, then someone had a great deal of insight and money to have done that back in the late '70's when I saw them.