well you remember wrong.,. the prefix letters stamping has everythign to do with the ratio, and whats what these letters stampings are for exactly... most GM truck axles werent tagged like many of the dana and ford axles were. a tag on the fill plug on the posi/lim slip equipped axles was about the only time you saw a tag on the 12 bolts.. otherwise no tag on these axles.
the HA prefix stamp code is for the ratio
HB,LL,RA.TR,TT,TU are 3.07s
HA,HJ,HQ,MF,TQ,MI,MK are 3.73s
HK,HL,HR,ME,MG,TS are 4.11s
HC,HN,MU are 3.73 Posi
HO,HP,JQ, are 4.11 Posi
the four numbers are the julian date.
0426 is the 42nd day of 1976
call local dealer parts counter with that code and year make model and they can tell you what it was from factory... if its the original axle anyways.. it was a 1976 3.73 open carrier
and in these 12 bolt truck axles you would rather have open diff than the G80 gov lok, AKA grenade-lok