F is a 4 row machine, M is a 6 row machine, l is an 8 row machine. In the rotaries the R40,42 is the 6 row, the R50,52,55 are 8 row machines. (Refering to 30" corn rows.) As I recall the F3 could also be fitted with a 6 row head.
The M series is the next size up, then the l's. I ran an M for years with a 6-30 head and did great,but went to the l to get the hydrostat that i didnt have.