It is an N frame.
All the 20's are N's.
24 is a 44 mag
the 25 is either a 45 colt ot 45 ACP
the 28 and 27 are 357 mag
and the 29 is a 44 mag.
Try Brownells they have a selection of wood, rubber, and other materials for grips already to go on your round butt.
Some will even give you a round grip to square grip profile.