[GreenKeys] SB-2244 TTY patch panel

[Nick England]

I have acquired a Navy TTY patch panel SB-2244/UG.
http://www.navy-radio.com/rtty/misc/sb2244-ug-01.jpg
It has 6 TTY loops and looks much like the common SB-1203 or SB-1210.
However, it seems loops 1-3 have signal current adjust and metering,
but 4-6 have "STEP LOOP" lamp indicators.
It is red so I assume is used on the classified side of crypto gear -
but what is a STEP LOOP?

Any info, manual, etc., greatly appreciated.
cheers,
Nick K4NYW
www.navy-radio.com