to further this discussion;
(because I honestly missed the other parts of your query)
There was (as I have understood it) a water pump change during the 1969 model year which necessitated a TSB to all in use vehicles;
The "snout" on the later 1969 water pump became longer.
The "1970" model year parts were casted in 1969, and would carry the casting number for the 1970 model year part.
You are either misreading your casting numbers, or you have a replacement/aftermarket item in place.
GM used a four digit (occasionally reduced to three digit) date code in the following format:
MDDY
Month = letter
Date = one or two digit date
Year = single digit; final digit in calendar year.
There were also often signifiers indicating if the part was cast on the day or night shift, which casting mold was used, as well as signifiers which elude my understanding.
The following image is the correct part for a 1970 model year Pontiac:
A dead give away for a water pump having been rebuild is the pulley end;
if it has two bolt patterns, it's not OE - it's been remanufactured.
Here is an image of a later water pump with the common casting number I previously mentioned:
There should be two long bolts, and two studs that hold the pump in place;
IIRC the studs use fine thread nuts, and sometimes people have replaced the studs with long bolts;
The long bolts can seize in the block, so take care - but worst case is that you may have a little more work to R&R that water pump (which will include a timing housing R&R).