Don's right. The eBay tank you posted the link to is the Starfire tank. And no, I haven't seen the large A65 tank like my Blue one at British Only. I've never seen one for sale, other than the one on the bike I had to buy to get mine.
The two most impossible tanks to find seem to be an original Spitfire Home Market tank and the one like my blue one. And at least you can get a reproduction Spit tank from Burtons. Would this one work for you?
Any dry frame tank will fit any dry frame. Only the rubbers, center bolt, and stabilizer bracket varied.
But one caution, when dealing with the early large home market tank, single and dual carb versions exist. The dual carb can be installed on a single carb bike, but not the other way around. The dual carb tank has cutouts for carb clearance and for some reason, I seem to remember the petcocks are located slightly differently.
So Jim how close is the spitfire tank profile to yours? Is your blue tank the same one in Bacons book that shows the 1970 home market tank? Would a 65 lightning tank work on a 1970?
The Blue Lightning tank is a very different shape than the Spitfire fiberglass tank. I think the blue tank is the one pictured in Bacon because that's where I fell in love with it.
The price of the Burton tank is actually quite reasonable, IMHO, 225 pounds, or about 350 US dollars. And they now claim that these tanks are ethanol resistant, so they shouldn't need lining. That's kind of a bummer for me, because I bought mine from them a couple years ago, and will probably have to line it to use it.
OR military B40 or GS Scrambles and there were a couple of others that were made from the same upper pressing and had decals so did not need a relief in the side of the tank. If it was going to sit high and did not need cut aways to clear the engine / carbs etc then BSA oft fitted strange bottoms to tanks.
Definitely NOT WDB40. Those are bigger...same body as the big A10 tanks but without badge indents. If it wasn't ever chrome, most likely '70 starfire (US) or Fleetstar(UK). Funny thing is, A10 tanks fit right on the unit singles because the cutout at the back clears the seat rails...
A smattering: '53 Gold Flash '67 Royal Star '71 Rickman Metisse '40 Silver Star '37 Rudge Special sixtyseventy Lightboltrocket road racer...and many more.