according to my grandpa who was a certified gm tech back in the day when i bought my 54 3104 he called it a heavy half it's got 10 leaves and the the standard had 8 leaves but it was an option you could get on any model you just had to order that spring set. those are aftermarket overload springs my 54 has them also they work too i have filled that bed with sand and gravel many times and with the 4 speed it was no problem one has a gross wt of 4200 with the 8 leaf springs and 4800 with the 10 leaf that's what my shop manual says also
