Google I found a table excerpt from the new SA design book 67 72 chevy trucks
Has a table listing Diff stamp codes listed HC states 3.73 posi 1967
Gmc Pauls also hase code Hc listed as 3.73 posi no year listed
If you choose look inside on there site page 103 I see they List it in 1969 so that code will be for 67 to 69
https://www.cartechbooks.com/how-to-...1967-1972.html