may not be an HO block.
from another source
The casting code for early 71 455-H.O. blocks is 483677. The casting code for late 71-72 455-H.O. blocks is 485428. The casting code was also 485428 for the 455's (non-SD) for both 73+74. The block codes for the H.O. blocks are WL, WC, YE, WM, YB, and WD.
Pontiac must have used some of the same 455 blocks with 4 bolt main caps installed (not just drilled for 4 bolt caps) in the non-H.O. cars and just topped them with cast iron intakes and D-port heads. For 75+76 the 455 casting number was 500813, and as you know, 76 was the final production year for the 455.