Someone has offered to sell a Beta 58MR. I don't know of that model. Is there one? If so what is it compared to a Beta 58A?
It is the first version of the Beta 58. "M" for matte finish of the ball grille; "R" for recycleable outer box.
The prime difference is that the Beta 58MR did not have an output transformer; the Beta 58A does. The two models sound slightly different.