Seems like the R-390 has been left out. No, I do not mean the R-390A. The
R-390A was designed to be a lsee expensive version of the R-390.

The R-390 uses LC filters, which can yield better audio than the 390A, with
the mechanical filters. The mechanical filters also inserted a phase shift
of the signal.

Best to have one of each.

Colin K7FM