72 318 Scamp. Low oil pressure 1 minute after startup.

Yep.... now that the hole location is known, that blocked hole is bad juju... that is one of the main flow passage from the block up into the head. Looked like casting flash and it could be a batch of it that got loose... looks like you poked it out.

That block needs a good cleaning for sure. Can't say if it was not cleaned or if it was quickly cleaned and the subsequent running with fresh AF loosened up more crud.... had that happen to a block that spent 2 days in just the rust tank and then a day in the hot tank... even that did not get it all out, and the later operation with fresh coolant brought more and more out. Needed coolant changes after the rebuild. And yes, the AF may never have been changed there.

The bores look awfully shiny to me (too smooth), but flash pix can be deceiving and exaggerate that factor. Looks like someone got carried away with a ridge reamer in the last pix of the bores, or there is a very strange ridge there.