OK - the later D/LD models had a filler/level plug on the small rectangular plate on the very end on the transmission casing, the early ones didn't. They have to be filled through the hole in the side where the speedo drive fits - use SAE140 grade gear oil, and the correct amount is 1/4 pint.
The gearbox uses SAE30 oil (or any 20/50 multigrade car engine oil), and the amount was increased on later D/LD models by moving the level plug to the side of the clutch housing, rather than underneath. The original amount was 1/2 pint, but the capacity was increased to 1.25 pints when the level plug position was moved. I'd be inclined to put in around a pint if yours is the early type to be on the safe side - half a pint doesn't allow for much leakage......