Interesting topic, would be cool to find out the end result if the mystery is ever solved. I looked at it as well as possible of a "Blow up" tank, but for many reasons I dont think it is. It is NOT inconceivable that someone did make these as a repop or replacement and not just in India.
I got some wonderful metal work done on my 1948 Speed twin tank and 2 other bits by a local craftsman who is not in the MC trade but took them on as an interesting challenge. But sadly,, metal work takes a toll and he no longer accepts jobs.
But I know a few other folks who also roll and form steel and alloy, including a local engineer for a well known international manufacturer of trucks who in his spare time built a custom hand formed steel body for a customized car.
But certain aspects of those tanks point to die forming, 2 ways to do that, one a short term limited production buck (Often hardwoods) or a hardened steel form buck and a plate pressing (Very expensive and still only good for so many pressings)
I will be submitting a tech article for several clubs newsletters this winter on the dangers of dent removal and the right and wrong ways to do so. I have several wonderful examples to use that were victims of DPO Bodgery. One was a Norton tank owned by of all people a structural engineer. (I wouldnt live or work in any structure that guy worked on) He should have known better, but tried to blow out dents with compressed air. Tank looks like an elephant stepped on it.
I will say in summary, DO NOT DO NOT DO NOT DO NOT, NO NO NO NO.........NO! use compressed air, frozen water or hydraulic pressure to remove dents.
Here is why, The dent will be the LAST part to pop out or return to shape. Learn mettalurgy and its characteristics. When a dent is formed it is stretching and distorting the metal which in effect makes it HARDER than the surrounding parts. You CAN anneal or soften the metal and often required after metal forming or hammer work. But the dent is harder and therefore the last item to move. Now, if you made a reverse die, you could in theory use compressed air or hydraulic pressure because this would hold all the other surfaces in check and only the dent would have the opportunity to move.
In order to do proper metal repair you have to have a good grasp of the dents creation and then reverse it in that order, There is some really good you tube videos on dent removal, metal shrinking and metal forming. Ron Covell has some excellent books, videos and seminars out as well.