[Rocky Rotella]

Thanks for the extra images!

I’m wondering if this was a casting run that Pontiac specifically produced for a class racer (or racers) running a 1967 Firebird or GTO who needed a 9786133 block for class rules. It would have likely taken someone with an inside connection to get it accomplished.

[Rocky Rotella]

Here’s another interesting SR block presently on eBay.

https://www.ebay.com/itm/124991349056

Pontiac created a service-replacement 1966 389 fitted assembly (part number 9784021) using a typical 1968-1969 9790071 400 block casting and combined it with cast 389 pistons (notice valve pockets) with complementing valve reliefs machined into the tops of the cylinders.

It's NOS to boot! A 66 GTO guy could snap it up. He might have a hard time convincing a judge that this particular 9790071 block is a correct SR block for the application though!

[grandam1979]

This SR block is ground different than any I have seen with the top of the numbers left.

[turbo69bird]

I always say never say never. I remember when people used to say Pontiac never made a shift light tach, or an aluminum block, or RAV dual quad engine, GME 428 RAV block- or special gear ratios. Blah blah blah. All have been proven false. They made all of that. I’ve also seen a RAII block with no casting pad at all cast C118 do a search there was one posted on here as well