HP= High-Performance. Yep the early blocks didn't have the "352" identifier. None of the 406 blocks I've ever had has had that. The reverse "105" blocks are all MCC (Michigan casting center) castings starting in the early 70's. Your 406 block was cast at Dearborn (DIF) and to me are some of the best cast blocks I've ever seen. If you ever tear the engine down I'd be interested to know how thick the cyl walls are. Being the 406 was the test mule for the 427 some of those blocks did have thicker 427 wall cores cast into them. Your may or may not have the crossbolt bosses. Whats the date code off the block? It's above the oil filter adapter on the side of the block. In 64 Ford moved the DC location to below the fliter pad.

G.