I'm assuming it's a Salisbury axle and hasn't been changed over the years?
Have a wee peep here to see what it looks like....later rear axles/ Discovery axles look the same as a front axle diff.....also you see all Defender part numbers on this site,for a spot of light reading lol.
There's two different seals....I'd order both and replace with which one you find in there.
AEU 2515 is a leather seal(soak overnight in EP90) which is from the early axles..probably yours
STC 4401 a later rubber oil seal with a mud shield.
Go for the make 'Corteco' or 'Bearmach' or even Land Rover (seals made by Corteco also) and not Britpart IMO
All the best