It was difficult to assess size of these "new" stampings. I don't think they were provided by AMC - possibly by the foundry casting the "malleable lugs", as AMC called them. I know the engine lugs for G12/G15 frames were cast by an outside company and given part numbers by them, I guess that's true for all the lugs used on these frames.
What a puzzle to prepare the lugs and assemble it all with the corresponding tubes!
As for the hypothesis of a replacement frame - if the original frame broke, it seems unlikely the engine broke at the same time. If it broke, wouldn't it be more likely to restamp a new set of crankcases and a replacement frame while spare parts were available, using the genuine number? If the engine didn't break, even more so!
Did AMC / N-V provide replacement frames at all? I have seen loose rear loops (from the Berliner Bros. warehouse, actually), but never a front frame / "diamond". I believe there were some restrictions on the sale and registration of chassis replacements even in the 60's, otherwise anyone could grab an existing frame number and have a "duplicate" bike registered. P11 production ceased in Nov. 1968, and I doubt there were any "loose" frames available (if at all) following the demolition of the AMC factory and warehouse in the autumn of 1969. Of course, it is possible that a spare part frame came from the Berliner Bros. sometimes in the 70's.
Have you tried to backtrace your frame's history by talking to the previous owners? It might be an interesting exercise.
-Knut
