The small homebrew chassis  plugs into the ART-13 837  oscillator tube socket and provides simple crystal control.  The plate cap for the 837 is attached to the insulated post of the plug in chassis. Not my design but clever. Perhaps there is a CAP or MARS bulletin with the description.
BTW is you have a CDA-T chassis plug in for the ART-13 I've described a simpler installation using a Jones plug.
K4CHE
http://k4che.com/ART-13%20CDA-T/ART-13%20CDA-T.htm