SeriesII transmission - stupid question

Hi folks,
Looking for a bit of advice. My car seems to be leaking oil (I know they all leak oil everywhere). When I say leaking I mean that some of the underside of my car looks like it has been jet-washed with oil. Having investigated I have found what looks like a drain? plug missing, but to what I don't know.
The first pic is from under the car of the offending jetwash nozzle and 2nd my main gearbox for good measure (I know that the filler drain plug is out of the gearbox - I just did that)
It's in the casting immediately behind the engine sump and I guessed it was the front of the main gearbox Referring to my Haynes manual I can't find reference to a plug this location.
Two hours later, transmission tunnel and floor out I have discovered that I have the 'later model' transmission despite the fact it is a 1959 series II. I have now checked the oil in the main gearbox and its at the right level and clean!
I have checked the engine oil and all's fine (though refreshingly dirty). I haven't checked the transfer box yet on the basis that it is further from the engine than the main gearbox.
Having justified my title of novice can anyone give me a pointer?
Thanks
Steve

plug is a wading/drain plug for flywheel housing which shoulkd run oil free but your rear main oil seal is leaking ,when you say later ,how do you mean

Thanks James,
'Later' is just an expression I stole from the Haynes manual - Land Rover Series II, IIA and III 1958 to 1985
Under 'Routine Maintenance' there are two pictures of transmission oil filler and level plugs. The first refers to (earlier models) which I assumed mine was but I couldn't match up the filler cap, dipstick or anything. Then I spotted the picture labelled (later models) and my drain plugs and filler level plugs match those positions. My car was rebuilt on a new galv chassis in the 90's though they reused almost everything and I think the gearbox pre-dates that. It doesn't have synchro on 1st and 2nd.
By chance I met the chap who owned the car in the 70's and early eighties and he told be he had put the Fairey OD on it which it still has.
Steve

first had filler cap on top and dipstick ,next had filler cap on top and plug on side ,next just had plug on side,can be converted to synchro all round though

James,

Mine's the next then - filler cap on top and plug on side.
Thanks

Steve