A picture is worth a thousand words as Roland said:
The small No 1 non reversing Clockwork Motor
is the basic predecessor to the post war No 1 reversing Motor and similar
in size, but with the 'silent wind' mechanism. Available in red then blue.
The No 1a Clockwork Motor is the descendant of the original (No 1) Clockwork Motor from 1912 and larger than the No 1 non reversing, with a 'ratchet wind' mechanism. Available in nickel, red and blue. Binns Road tried to finish with this Motor when the No 1 non reversing was introduced in the early 30's but apparently the customers still wanted it, so it was retained as the No 1a. The No 2 Clockwork Motor is a heavier version of the 1a, similar in dimensions but with a much larger and more powerful Spring and hence wider between the side plates. Available in red and blue.
There are many more variations in these Motors. The early post war No 1 Reversing Motor was first fitted with the 'silent wind' main spring mechanism from the pre war No 1 non reversing Motor. It also required a 'T' Key (smaller square). The later (1949?) No 1 Reversing Motor was fitted with a new ratchet wind main spring mechanism, using a 'B' Key (larger square). The side plates are slightly different, to accommodate the two completely different mainspring spindles.
The nickel motor shown is pre 1922 or thereabouts.
In 1922, the bottom screws and pillars on the front (or 'dial' plate in clock
terminology as it is known from the drawings) were moved into the bottom row of
perforated holes - the left hand one being in the second hole in and the right
hand one in the third hole in from the corner. This was done to give more room
for the spring and a longer run in consequence. Together with a fifth screw
about 3/4" below the output spindle, the motor remained substantially
unchanged from then on up to the 1960's French versions which were probably the
last 1A's. Maroon versions appeared in about 1927, red came in about 1932 or
1933 and blue in about 1935, with black crackle briefly after the war. There is
enough evidence to claim that the 1A continued in red in parallel with the blue
up to and just after the war! Unlike all the other motors!
Post WW1 the side plate screws are all 5BA, an oddball size if ever there was one!
Both the No 1a and No 2 Motors were briefly available post WWII in black crackle finish.
Photograph courtesy of Roland Jaggard.