D2 Oil Clutch ?

OzCat - It's highly unlikely that your tractor has an oil main clutch. You can ID the oil main clutch by the following features ..

1. It has a dipstick, that sticks up through the floorplate ..
2. It has a double gearset oil pump, externally bolted on, high up on the RH side of the clutch housing ..
3. It has a suction strainer for the oil pump, bolted to the bottom of the clutch housing by six bolts ..

Your tractor is a very late D2, being built around mid-1956. The D2 oil main clutch became available as an option or "attachment" in Cat parlance, during early 1956.
Cat offered a "field changeover" oil main clutch that could be retrofitted to D2's back to 5U16128.
I understand that very few D2's were actually fitted with the oil main clutch .. a figure of only around 800 D2's, I seem to recall.
Your tractor may have been one of the rare approximately 800 tractors fitted with the oil clutch, but I think it's unlikely.
If your tractor has been running around with no plug in the clutch housing, and it was an oil clutch, it wouldn't have lasted very long.

The dipstick for the oil clutch would be towards the rear of the shaft you're trying to drive down. You said the drain plug is out, if you can see up into that hole and see a screen, it's an oil clutch. If there's no dipstick or no screen, then it's not an oil clutch.

The procedure for pulling the clutch cover is different for the oil clutch and dry clutch, if it is an oil clutch, the clutch lever shaft is driven down a little after loosening the control shaft lever that is down in the clutch compartment. if it is a dry clutch the clutch, the clutch control shaft assembly, comes up and out with the cover, after loosening the the clutch yoke, on the shaft down in the clutch compartment. if you post a picture, with the clutch inspection cover off, we can tell for sure and post the proper instructions for that clutch.