[Baron Von Zeppelin]

Late April to Early May on a Pontiac Plant car in that time zone.
By your invoice date of May 12

I think your current 1966 block is someone's attempt at a restamp.
Or an SR from a different car back when.

It's not factory stamping at all . But possibly haphazard stamped by a dealer tech.

Outstanding pedigree !
Black - TriPower M20 3.90
Congrats !

[Baron Von Zeppelin]

I have had - I think three 76XW engines - in distant past . all sold long ago
All were dated D in April , so that factor might work in your favor as being in the sweet spot.
Don't pass up ANY authentic legit factory stamped 76X or 76X9 block/engine you find , regardless of the date code.
There were slim pickings on those even 40 years ago.

76X could transform into 76X9 by a magic elf . fwiw

Highly educate yourself on what authentic 1964 block stampings look like. Before a purchase.
Can be any 1964 Pontiac V8 engine to use for comparison , so long as it's untampered.
Catalina Bonne .... even a 326

The one you have now is not a proper example.

All 1964 model year blocks will have the application code stamped ABOVE the EUN . Not below it.
They did get the orientation right on your current block , but that's about it .
Go on Franks and look at any/all 1964 blocks he has - hopefully he pictures their ID stampings.

[4dblnkldude]

Great advice BVZ!! Thank you, I may never find one.