Tyre size NORTON 1951 ES2

Books give 3.25x19 front and rear.  Inters had 3.00 which in theory will improve steering.  That's probably because Inters did not have interchangeable wheels.

The modern recommendation is to not fit oversized tyres to narrow rims. With the standard WM2, a 3.25" is fine.

After all those years anything could have been fitted.Other Nortons had 21"x 3.00 ,measure what you have.

... on my 1952 ES2 and it handled fine. Mitas.