I had two different panel tanks on my 1948 Square four at different times, the first being a "500 Single" sized tank and the second a Square Four sized tank which came to me from Canada. Both had the bolt fixing for the kneegrip plates rather than the stud arrangement.
Now it is perfectly possible that I am wrong, but it is also possible that Ariel didn't get round to changing the parts lists until some time after this particular change had been made. After all, it was pretty minor in the overall scheme of things.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.