Rex. Another conundrum.
For comparison, an early Plunger tank I have has lugs at 21" centres, which match the early plunger frame to hand. As far as I know the frame lug forgings and top frame tube remained the same, but further enlightenment from other sources is always welcome.
My later rubber mount tank has longer slotted lug at the front, believed original and unmodified, giving far more adjustment and leeway than the early tank, which has smaller bolt/stud size holes.
Swarfy.