So, Zack sent me a pair of the early RRC seals, P/N 571890. These are definitely not the right seals for a Defender swivel. The inner and outer diameters are the same but they are several millimeters deeper and don't fit.
However, I looked more closely at the original seal that was on the truck and got the part number: FTC3401. Some catalogs show this part superceding to LR059968, however, this is not correct! The LR059968 has much shorter lips and won't provide enough sealing against the early style swivel ball. If you put the LR059968 on an earlier swivel like the one I have, your swivel grease will immediately drain out.
If you come across this thread, the part you need is the real deal original FTC3401, not any successors.
Here are some photos comparing the two.
Side-by-side, the two seals look the same:
Here is the old-style FTC3401. It has longer lips than the newer style:
This is the new "equivalent" LR059968. This does not work on the early-style swivels because the lips are not long enough: