[Collins] KWM 1 power supply question - Mystery Jumper

[Greg Mijal]

Howdy:

My recently acquired KWM 1 has been a tough one to mate to my home brew power supply and wondering if anyone knows the answer to this situation.

Previously:
All filaments light up ok and 260 vdc applied to the B plus line.  Current draw is correct per spec's but no receive, no nuthin'.

Today: I spotted something in manual that does not appear anywhere else in the manual.  That is, the AC power supply schematic shows a jumper from pin 20 to 21 on the connector.

I put the jumper in and the receive began to give the standard hissing sound.
Looks like the jumper enabled something in the radio but the Collins manual doesn't seem to have an explanation.  
Can someone tell me what this jumper's electrical function is?  If I missed it and it's in the manual, what page do I go to?
Thanks
Greg
WA7LYO
Kinston NC