This information appears courtesy of Retina camera specialist David L. Jentz
 

Retina folding camera identified serial number series (1945 -1977)
 
 

model and type number	Serial numbers
Retina I (type 010)	50007 - 99795, 150257 - 228646
Retina I (type 013)	229470 - 237970, 258283 - 361805, 400052 - 404949
Retina Ia (type 015)	400093 - 400402, 405049 - 893831
Retina Ib (type 018)	50001 - 199894, 450629 - 460320
Retina IB (type 019/1)	462034 - 481599
Retina IB (type 019/2)	481600 - 503099, 50219 - 82687
Retina II (type 011)	100224 - 149133, 250160 - 258313
Retina II (type 014)	258283 - 361805, 400052 - 404949
Retina IIa (type 016)	400093 - 400402, 405049 - 893831
Retina IIc (type 020)	200032 -299762, 510241 - 753513
Retina IIC (type 029)	759812 - 761362, 76653 - 767011, 50001 - 63324
Retina IIIc (type 021)	200032 - 299762, 510241 - 722683
Retina IIIC (type 028)	753845 - 755529, EK756296 - EK758715, 761694 - 763835,
as above continued..	50001 - 103032, EK800009 - EK806823
Retina IIIC (type 028/N)	997xxx or 998xxx embossed on camera back leatherette

Back to first serial number info.