Locate the manufacturer's marking as indicated in the below picture(s).

EATON/FULLER (FS4000 MODEL)

EATON/FULLER (FS6000 MODEL)

EATON/FULLER (FR MODEL)

GM/MUNCIE

MACK (T SERIES MODEL)

MERITOR/ROCKWELL MODEL

SPICER/TTC (ES MODEL)

SPICER/TTC (PS MODEL)

ZF FORD & GM APPLICATION MODEL