[Boatanchors] Northern Radio type 178 AFSK converter

[Nick England]

I recently acquired a Northern Radio type 178 AFSK converter - does anyone
have a manual or other info that I could borrow or purchase? 

It is labeled as a Twinplex Converter  - if this means it is a four-tone,
two channel decoder, is this mode still in use by anyone?
I've added some photos of the type 178 and started a web page on Northern
Radio RTTY gear at
http://www.virhistory.com/navy/northern.htm

All comments, additions, photos, links, corrections, etc. are welcomed.
cheers,
Nick KD4CPL
www.virhistory.com/navy