Interstate petrol tanks

I notice that Les Emery at Norvil is advertising a 6 & 1/4 gallon petrol tank for £695. It is described as 'rare'. I have one of these and have seen plenty more. I think the smaller interstate tank (the narrower, taller one which holds 5.25 gallons) is rarer. Does anyone know the ratio of large to small tanks? Or even the proportion of Interstate to Roadsters?
(All capacities are in English gallons).

Thanks, Mr. Rags

I think whats rare is the price!!