My 13.75 gal genuine Auster tank had the filler on the port side. But given Auster's ability to make anything they needed pretty much that day and their tendency to never make a series of anything who knows?
Even the wing tank capacities can be different (a Terrier I knew of had on 11 and one 16 gallon tank!!!
At the end of the day the only way to be sure is to fill them ad then run them down until the engine starts to die and then measure what goes back in.
My belly tank would have virtually nothing left in it - but I've heard of others where their might be well over a gallon in the bottom.....