A square butt K-frame at it's widest point, is 1 13/16" across, or 45mm. Don't know about a K round butt. I don't own one. The relief in a set of grips is the same amount, measured at the widest point.
There is....most times I can tell by looking at the backside of them.
The distance from the washer to backstrap edge on K''s
Is right about 1/8" on N''s it is 3/16+