And the transfer box is probably filling the OD because it too is over-full, having had the main gear box's oil dumped into it - oil migration from the main box is incredibly common.
I found that my rebuilt gear boxes all had the same fault, including the one from Gen Parts in Solihull. The main shaft rear bearing carrier had been set in the casing dry. Without the red Locktite to set and seal the carrier, oil was able to make its way past the carrier/casing joint, bypassing the main shaft seal. Now that I have rectified that fault, I no longer lose any main box oil other than the tinest weeps from the selector shaft seals (to be tackled next with doubled up o-rings in place of the square section oversized seals). The Locktite also stops the bearing carrier spinning in the casing.