Because period nomenclature differs from modern terms I will explain some leather types here.
There are some types of leather which are not available any more so I list its modern equivalents.
1. bridle leather = vegetable tanned cow leather between 2.5mm and 3.5mm thick
2. harness leather = dito but 4mm+ in thickness
3. waxed flesh/upper leather = bridle leather, dyed and waxed on the fleshside. The name upper leather derives from its usage for boot and shoe uppers.
4. buff leather = this is an oil tanned cow leather. It does not have a visible grain side left as this was "buffed off" in the tanning process. This leather is extremely hard to obtain because there is barely a tannery left that is making acceptbale buff leather for belts. It has a yellowish color. This leather is pliable.
5. sole leather = oak bark pit tanned cow leather
Buff leather was the typical leather used for belts and shoulder straps before the Civil War and at its beginning. Due to the difficult tanning meathod it could only be produced during the spring and fall months when the climate conditions were adequate for the oxidation process of the tanning. The army already experimented with waxed leather in 1857. This type of leather was used till the middle of the war when bridle leather took over.