Lets get this out of the way first , there is no missing tube (the empty tube socket on the back) this socket was used for a plug in filter cap. The filter caps are new and under the chassis. The little screw in neon bulb socket has zener diodes (inside) for regulation instead of the neon bulb.The SA39 will not work properly if you don't have the right neon bulb in A3/A4. (or change it to zener diodes)
This is a completely rebuilt excellent working Gates SA-39B.
I have a manual and schematic for the SA39B . I have 2 SA39B's this one and a Black and Silver unit that have been rebuilt and are my keepers. I have decided to part with this one .This SA39B is one of the nicest units I have ever had both cosmetically and electrically, it is completely rebuilt with the addition of variable release and threshold.
We add all the factory changes that show up on the last SA39B schematic , plus adjustable threshold, selectable release times and . you can see it on the SA39b at the bottom of the page. . We use the same holes that are already on the front panel and add the above functions."