Overbored 390 block used in '64 Fords and Mercurys. Not real uncommon for the year, but still a good solid block to have. I have the same block that came out of a '64 Mercury. Built it for my '68 F250 Highboy, ran it for over 20 years, now it's going to be a 445 stroker for my '70 F350 towing truck.
It's just my guess, but I think Ford either used leftover blocks from the 406's used previously in '62-'63 ( only difference being drilled for hydraulics), or used the casting molds to cast these blocks for part of the '64 model year. Later changed them for weight and savings on iron. That explains the thicker cylinder walls and crossbolt knorbs. But like I said, that's just my guess.