Rotomo heads are a full dome on the external.
Special heads are square around the base with a half dome on the external.
Fin positioning and count are the same.
The later/last 125 head for utility, standard and sheerline are a totally different fin style with center plug hole. They still fit and will work with the earlier engines.
So 4 heads,
Model 1 Rotomo- Thicker fins
Model 2 2a 3, up until early 57 approx has Thinner Fins same style as 1
Model 4, Auto, 5 Special and Model 8 Standard up to Mid 60 approx. Same fin style and count as 1 and 2 but with a half external square based dome.
Model 6 9 and 8 after mid 60- approx, has centre plug hole and different fin count and style.
Yes the later head will fit an earlier rotomo as long as its 125
