I pasted this off the CP site. In my opinion, this is the most interesting statements so far on this subject:
Written 3/9/05:
"SOME of the PHS docs will show the Motor Unit No., but most do not. '64 Fremont docs do, '64 KC docs do by virtue of having an imprint of the factory produced warranty Ident-O-Plate on them. The '64 Pontiac plant doc does NOT show the m.u.n. Motor unit nos. can help determine "matching nos." to a degree even if the warranty booklet is gone and the PHS docs don't include it. A careful study of m.u.n.'s can help determine if a particular m.u.n. is POSSIBLE for a given car build date or not and is a LOT more precise than a block casting date. But it takes a TON of m.u.n. study to sort out actual engine assembly dates by m.u.n., so don't expect anybody to have casual knowledge about this"
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