Hi Jeff,

'Fraid you aren't extrapolating correctly.
Your bike's cylinder head "date stamp of 62+" simply indicates it was
cast some time during the 1962 calendar year.
Significantly, 650's changed from pre-unit to unit from the '63 model year, so
during the 1962 calendar year. Aiui, one of the pre-unit-to-unit changes was the ninth bolt through the cylinder head into the block, between the bores? If so, and your bike's head/block has the ninth bolt, your bike does not have a "'62 [
model year] head", it must be a '63 model year or later unit head.
On a complex part like a cylinder head, the difference between a casting and a part ready to be built into an engine is the machining of the casting - pre-unit and unit 650 heads
might be the same
casting (with "a date stamp of 62+") but the casting would be
machined differently depending whether the finished part was intended for pre-unit or unit engines.
Then why would "part number 70-6000 (as shown in the Healy article), for model years 65-67 ... be incompatible with [your '63-on(?)] head or any other future replacement head outside of model year '66?"
Firstly, digressing slightly but importantly, in the Healy article, afaict "1967" under 70-6000 and "1968" under 70-9349 are misprints. The PRT changes coincide with the change from pre-'69 70-3547 white oblong-section seal to the '69-on O-rings (70-7310 superseded by 71-1283) and, as the texts under the article's two
cylinder head images say, the seals change happened '68-'69 and what M'ick called 'the "socket" for the PRT' changes ID significantly.
So afaict, "part number 70-6000 (as shown in the Healy article)" should read, "650cc 'B' Range 1965 to
1968".
So, unless your bike has an 8-bolt head, you're looking at a '63-'68 unit 650 head and '65-
'68 PRT.
Then the difference between alleged PRT lengths - pre-'65 70-3646 and '65-'68 70-6000 - is only 0.070". For '65, Meriden went from pre-'65 different 'white seals' top and bottom to '65-'68 the same 70-3547 'white seal' top and bottom - 70-3547 is ~1/10" (+-0.1000) thick, the superseded 70-1476 is ~3/16" (+-0.1875") thick, the 0.0875" difference being remarkably similar to the the difference in alleged PRT lengths?
So, if your bike has 70-6000 PRT
and the same-thickness (and -ID and -OD) white seals, the
total height of PRT
and seals isn't any different from 70-3646 PRT and different white seals; either combination works with any pre-'69 unit 650 head.
And, in the unlikely event your bike somehow does have a '62-model-year pre-unit head, exactly the same 70-3646 PRT-and-different-white-seals were fitted to them too.
John Healy is at least Coventry Spares, Podtronics and Tympanium; supplying retailers is what pays his bills; at past eighty years old, I suspect he probably likes some 'me' time too ...
Hth.
Regards,