I've got a couple of Subal port type 3 o-rings left over from my move (err, some years ago!) to a type 4 port for my ND800 housing.
One is black, one blue. I must admit I don't know what the difference is.
If they are any use to you drop me a PM. I'll happily mail them to you free of charge if you live in the EU.
It seems a pity just to throw them out if they can find a happy home.