['ol Pinion head]

There are at least two different cast factory assembled impellers for the 8 bolt pumps. The very early ones have abbreviated (short) vanes were used very early in 8 bolt pump production. Def in 194's.
Many of that style 8 bolt pump went through the mass rebuilders in the 60's & 70's & occasionally those early style abbreviated impellers ended up being rebuilt in later cast 8 bolt pump housings..

The 11 bolt Pontiac water pumps used one of at least 4 different style cast impellers.
The first for the "351" casting pumps, second for the "124" and "138" casting pumps.
The "296" casting pumps began to be used in mid '72 production & incorporated a different bearing/seal cartridge.
"296" casting pumps continued with normal length vane impeller up into the late '70's.
When the 301 was introduced for '77 models, the cast impeller changed to one with shorter vanes/less drag, thus
the longer vane earlier '296 pump cast impeller was not needed in the shorter deck 301 or 265 block.
Over the years, have found original cast 301 impellers do come in handy when rebuilding 400 & 455 blocks that have been hard blocked up to the bottom of the freeze plugs.