From what I can gather, the earlier types (2SJ26) (such as in your primary B2) were much more robust VFet than the (2SJ26A) "A" version as mine.
50volt is a huge voltage advantage on the Drain-Gate from the earlier non "A" VFets versions which I expect translates into thicker deposits and a more expensive manufacture.
Early 80’s Yamaha would have been in the closing stages of Yamaha VFet, 2SJ26A /2SK76A manufacturing, even with the introduction of this “A” version in the late 70’s, which appears in latter B2’s, I would suggest this as a stop gap model, to keep costs under control.