TNC2.16
I haven't actually tried these to verify them.  Also, there is a CMOS
82C51 available, and an NMOS 8251.  According to the manual, the NMOS
8251 is used; although most of the other parts on the board are CMOS.  I
don't know whether the IC kits they provide actually supply the CMOS or
NMOS version.

In addSY light oks like maybe the person who designed the
receiver intended for these to be external controls, and then they put
them inside when they packaged the receiver.

What's interesting about these controls is that they, and the IF shift
control (and another control used in aligning the receiver) are read
through an A/D converter.  They actually serve as digital inputs to the
microprocessor.  It's unfortunate that they didn't use one of the
channels on the A/D converter to read the signal strength meter (so you
could find the signal strength (AGC level) through the serial port)
instead of using them for these relatively minor functions.  EEB in
Vienna sells a modification for the R5000 which allows their CRIS
computer interface to read the AGC level, but I don't know how it works.