Just Googled "M3 variations" and came up with this:
M3, first type SNR: 700,000 - 785,800. 1954-55. double stroke.
no field selector lever. rewind knob with red slot. glass pressure plate. frame counter: black to 740,000, white thereafter. triangular strap lugs. shutter speeds: 1, 1/2, 1/5, 1/10...
M3, second type SNR: 785,801 - 854,000. 1955-56. double stroke. field selector lever, and thereafter. rewind knob with one or two red dots. glass pressure plate to 844,000, metal thereafter. strap lugs as above. shutter speeds as above.
M3, third type SNR: 854,001 - 1,206,999. 1957-68. double stroke to 915,250, single stroke thereafter. modern shutter speeds: 1, 1/2, 1/4, 1/8, 1/15... rewind knob with two red dots. strap lugs triangular to 915,250, round thereafter.
Here's the link to give credit where credit is due:
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00LdsU
Here's a link to info about M2's with/without rewind buttons and with/without self timers:
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00J5O9