Flavia Coupe Gearbox: 2 drains but one oil?

[markinnes]

Just read the Gearbox Oil thread. I'm confused too, but not for the same reason! I have a Flavia 2000 Coupe 820.031.2xxx from April 1970. It has the Four Speed 'Big Box' but with the single EP90 oil 3.8 kg plate [as per the Consortium tech page
[url][http://www.flavia2000consortium.org.uk/Technical/techhome.html]
Previous owner Mr Rieu says he changed the gearbox with another Flavia [unknown age/ series]. I have two drain plugs, one filler plug appears standard, and one filler ground off so it can't be removed. Is this what Lancia did as standard for the later single-oil four speeds, so I'd be right to use one oil, or has some-one mis-guidedly tried to convert a two-oil set -up to single?