[GreenKeys] Dovetron question

[Jim Haynes]

I've never yet seen two Dovetron units exactly alike.

Mine has the regenerator/speed converter TSR-100S    I have lots of
info on the TSR-100D, but nothing on the S model.  This involves a
slide switch on the PC board labelled S6 SDS Control with positions 
Test, On and Off.  There is a switch on the front panel labelled
SDS with positions OFF, (presumably ON) and RESET.

Does anybody know what this feature is?

Jim W6JVE