I know there are far more knowledgeable members out there when it comes to Cheddar stuff butI strongly suspect that the boilers were identical in fittings but they were made as two sizes. It would be interesting to see what the capacity came out at as that may help identify it.
Another thing is that some Cheddar stuff did have identification numbers engraved on them. Have a good look around the shell and see if you can find a number.
As for capacity, simply fill it right up with water and then pour out into a measuring jug. The capacity in litres may line up with one of the figures you quote. Lovely boiler though and well worth putting into a boat.