Miriam,
I just checked the screw holes on my garage 201-2s and one garage 15-91. The ones in the garage/ shop are to be fixed up and sold or are to be parts machines. (yea right, if I ever get around to selling any.)
Anyway, the 15-91 that is from 1935 and the 201-2 that is from 1936 have four holes. The three 201-2s that are from 1942, 1950 and 1956 all have two holes.
Possible a QB survey could answer the question: Does the year of manufacture determines whether there are 2 or 4 holes (affecting the ease of conversion to hand crank or treadle.
Cathy