My 63 engine had 426 TMPHC iirr but really there’s no difference in any block from HC to non HC, just what’s put inside
T stood for 1963
MP was maximum performance (max wedge I believe)
HC was the high compression version, 13.5 vs 11.5 I believe
At least that is what I recall from playing around with them back in the 80's.
You are correct about the actual block differences though, none.