Vintage Simmons Drums (UK) Users Group

Hi to everyone, and Happy Holidays?

Can anyone tell me what is supposed to happen with this input?  Is it a
THRU for routing and external signal thru the SDE?  Mine doesn't do
anything, and the manual (although mine is intact and complete) does not
explain the use of this.   Please help, and thanks.  I have utmost
respect for the SIMMONS users/wizards on this message board!

Christopher