After reading this thread, along with my own comments, I found Bruce Andersons's web site and posed the question to him about when the studs were changed to Dilavar, did he know the engine serial numbers when this change took place, etc. The following is his reply to me, in full:
"Sorry, no I don’t know when. I have never seen specific information on the
change. Remember tech bulletins were a secrete back then, they didn’t want us
to know that kind of stuff.
I was working on the cars on a regular basis back then and some of the 1977s
had the good valve guides and some didn’t. Also some cars had the Dilavar
head studs and some didn’t.
Bruce Anderson
www.911handbook.com "
So I guess some of us with 77's may be lucky, and some may not. My car was built in April of 77, so it was built some where in the middle of production. I heard that German model years go from August to August, so maybe mine is late enough for all the changes.