You are on the right track. The 101 Jr. and 101 Sr. production ended in 1946 when the 30 and 44 models were introduced. The 44 with 4cyl overhead valve 260 Continental was the flagship rowcrop model. The 44-6 was considered the "economy model". They were often shipped with very few options. Prior to the 44, the 101 Sr. was the flagship rowcrop tractor in Massey's lineup.
The 101 Jr. and Sr. tractors had 4 speed transmissions; the 30 and 44 series had 5 speed transmissions.
The 101 Jr. used the Continental F124 flathead 4cyl to start with, updated to F140 and finally to F162 by the end of production (bore was the only difference). The 30 featured the F162 throughout production. Both the Jr. and the 30 have a front frame that is about 6" shorter than a Sr. or 44.
The 101 Sr. used the F226 Continental flathead 6cyl, as did the 44-6.
So to sum up:
30 replaced 101 Jr.
44 replaced 101 Sr.
44-6 was economy model, most similar to 101 Sr.