In practical terms, Model A refers to a string scaling needed for a unique body style and then a unique string set.
Model B refers to an instrument style as opposed to model A, but the letter B was used in other ways within model numbering. There is a 15B chord layout and a B style chord bar design. B type chord bars have been used on model A instruments. In most cases, any reference to a model B will intend to describe the type of body style, versus a model A.
A prime distinguishing feature of a model A is the string end pins on the end of the frame, the pin cover often missing.
The most distinctive feature of the model B is the slotted aluminum string anchor bar set into the top. Any fine tuners would replace that, so the universal distinguishing feature would be the vertical, notched bridge pins in front of the tuning pins. Those are in lieu of the model A type bridge, which would have a wooden platform and metal bridge rod running in front of the tuning pins.
Model C is a type of chord bar, not instrument. Model D is also a type of chord bar, not instrument. They are both used to describe different sizes of felt. The model B body carrying model C chord bars is referred to as a model C, e.g. OS45C is a model B Appalachian.
The C is actually redundant if the model includes the number 21. The reason the C is included anyway is because that type of bar was briefly used on 12 and 15 bar models. Therefore the "C" did not always correspond to having 21 chord bars. The "C" then can be associated directly with the word [C]over, as in chord bar cover with 12, 15 or 21 holes. The OS21C then can make sense as a model number.
21 bars were never offered on vintage model A. However, because bar sets can be added or changed, one must see the body style to say whether one has a model A or B. The bar set is not conclusive.
The Model A Reissue is yet another variation. Although it can take a similar type of string, it really is unique in many important ways, differing from both the model A, model B, and Chromaharp. A separate string set is not always offered but should be.
Pictures are always best. The following links should be useful in being more certain of what type of instrument body style is under discussion. The objective is to be sure of type of strings or parts availability for example, not to precisely identify a more complicated model number. The question is "do I have a model A or B?"
Model A:
12 chord blackie
12 chord blackie view 2
12 chord bar set
Model A bridge type
Model B:
Early 15 chord model B with soundhole
1975-77 Appalachian as 21 bar example
Chromaharp:
15 chord Sekova
Model A Reissue:
OS73C Model A Reissue