The transmission rear cover is not an original IH cover, as made by IH. The top (and bottom) dowel pin was used with the cover for a rear PTO, to prevent any side-to-side or twisting movement of the cover/PTO assembly. The cover plate used with the PTO had an opening with 6 holes around it, 1 for the dowel and 5 to bolt the PTO bearing housing to the cover. There was a small blank cover to bolt onto the rear cover if the PTO and bearing housing was removed. These dowel pins can be rather hard to remove once they get rusty, so this transmission may have had a PTO in it at one time and when removed the dowel left in place and a cover drilled to fit the dowel, or a PO may have installed the dowel for another reason. Be interesting to see if the transmission top shaft is for a PTO.
The rear covers without the fill plug, used on the Original and a Farmall Cub, do have the same part number (350 836 R1) according to the TC-82 Parts Manual (Rev 6 Dtd 3-9-64) for the Original, and TC-37F (Rev 6 Dtd 8-79)for the Farmall Cub. A TC-37 (first issue, Dtd 6-2-47) shows the same P/N. The number could have been changed to an IH prefix number later, but if so both should still have the same number.
The P/N for the rear cover with the fill plug such as used on a 70/100 or later is 384 930 R11, later changed to IH-106394-C1.