Information from ISMACS:
http://www.ismacs.net/singer_sewing_...ist/index.html
27 class (USA) 1989-1913 Domestic Vibrating shuttle; rectangular bed, low bobbin winder; manual tension release;
14 5/8" bed. 8" to right of needle. 15x1 needle
127 (class) 1913-40 Domestic 'Improved' class 27. Vibrating shuttle.
Full size bed 14 1/2" x 7". 8 1/8" to right of needle. (Download manual) -
28 (class) 1890- 1918 Domestic! As class 27 but with
3/4-size bed (12 1/4" long; 6 1/2" needle to base of arm). Vibrating shuttle; manual tension release, no shuttle eject button. Originally 'VS3'. 15x1 needle
128 (class) 1912-52 Domestic 'Improved' class 28.
3/4-size bed (Bed 12" 1/4" x 6 1/2"); vibrating shuttle; high bobbin winder; automatic tension release.
So to review: 27 - 127 = full size machines, 28 - 128 = 3/4 sized machines.
Joe