It is possible for the '55-59 third member to be installed in a torque tube rear housing. If yours is a TT housing it will be mounted to a hinged bracket which should be welded when the when the later unit is installed. If your housing is bolted directly to the springs there is a good chance it is a '55 first housing.

Thanks for the info about 100-24P-Z15410 being the first truck with the open driveline. That was for the Pontiac plant anyway. I've often wondered when the change was. My GMC MPBs are a '54 and a '59 and neither give that info. I still need to find out when Oakland and St. Louis changed.


"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.