What Transmission is designated the HD 3-speed Manual

[rechinca]

Ok, first I am not at home for a few days and have been trying to do some research with zero luck (including the search function).

I have no way to look at the truck for several days and want to know what transmission was designated as the H.D. 3-speed with the 350 for 1971? This was spurred by knowing the rear seal is leaking and I want to replace it, but just doing some parts research some places specify not for HD. Looking to find make/model and I thought I had seen somewhere it was a Muncie, but of course I am search function challenged now and can't duplicate it for some reason.