I'm NEW here .. PONTIAC transfer lug can anyone get it right?

I just purchased 2. 63 grand prixs for parts as a package deal for spare parts for my 63 gp 4 speed car ..
the one has a fresh new engine that has never been started ' it is a SR engine
it has the transfer lug . 2 freeze plugs and is 2 bolt main with only 2 motor mount holes .
the problem is no one on the internet can get the **** right
the block is a 543680 at the distributor hole so it should be a 1963 389
but............... it is dated . sept 25 7 who knows what pontiac did with blocks sitting on the racks year after year. this engine is a SR CODE
MEANING the dealer installed it . the block has no production line ID numbers other
than the SR8 4748 9729 at the bottom of the block by the water pump inlet in a "T". pattern . most SR blocks had no ID numbers .. my question is why the late date on a pre 67 block .. confused in california ....... any replies welcomed

 

It might be a factory warranty. G.M. would actually have the dealers rebuild engines under warranty.Then re-stamp them back in the day. I've got a 396 like that.

 

Welcome from O-HI-O
The Rock