With so many "should haves" it sounds like an unstamped block with the right part number and date could pass as a HIPO? A block that was "missed" in stamping, perhaps never grease/paint marked, or as you've said with these markings washed away during a rebuild, this block in the topic ,though not likely, could be passed off as the real thing. Its the "common" pieces of the puzzle that we all look for, main caps, balancer, counter weight, rods, vin stamping, etc. thinking its a correct HIPO.
Always learning something new here.