If you are talking about PE250B, well they hold more not less. 12L. Same as 78 plastic tanks. Although i have come across some later 1980 tanks that held odd capacities between 11 and 12L when they were supposed to be 10.6L. I think the factory may have slipped a few left over 78 12L and 79 11.5L tanks onto some of the T models. Even 'Le Canari' in France found this with a few of his tanks too.
Clarke MFG could make a lot of money reproducing the PE tanks, but they wont even do 81-84 RM250/465/500 tanks even when many people contacted them. I don't know why. They have any competition. We just need to get them to see dollar signs somehow.
It would be a pain to measure my 400 tank.
It was in top condition,,,so I filled it with cheap engine oil and boxed it up from the light(into storage).
From memory though? 11.5 rings a bell,,,,,, did right it down somewhere?? ?? ?