Flat Field Vs Concave Field

As far as I know, it was only on the George V nickels.
Because of the hardness of the pure nickel planchets, the mint curved the fields at the rim which
allowed more metal to move towards the portrait.
The concave field planchets should have sharper portraits whereas the flat field portraits often look
worn even on MS coins due to the lack of metal.

1948 50c specimen coins have both concave and convex varieties (as defined by the obverse/portrait side). I'm not sure if that is part of your question or not. 1947 50c Maple Leaf CR specimens are also clearly concave on the obverse and convex on the reverse, which is not documented anywhere that I could find but perhaps because they all are. On the 1948s the convex ones show as weak strikes in the reverse shield and lion's paw (which is also seen on circulation coins), whereas the concave examples are well-struck in that area. In this image you can see that the portrait is sunken in, and perhaps not as clear, the reverse is bowed outward. It's very obvious in-hand.

I don't know if there's anything at the rim that defines these and I had not heard of the nickels. Does anyone have comparison images of the nickels?