400 "super block"

hey fellas

i was recently told my 400 had the "super block" numbers, so i went on 440 source and read the article, im a bit confused as mine is stamped as a 71' year block but it has the superblock numbers, and the article reads 72-73 have the biggest measurements, does anyone have experience with this stuff? ive emailed 440 source but havent had a reply, thanks