The B-10A service manual says:

“B-10A series models are based on B-10 series models.
B-10AR model is identical to B-10A model except that the B-10AR model has a rhythm section and that the B-10A model does not.
Major modifications from model B-10 are:

• newly developed NS speaker JA5101
• B type main amplifier
• new cabinet commoly used for both B-10A and B-10AR models
• new YAMAHA auto rhythm unit equipped in B-10AR models.”

Information provided by Markus Sigg