Q4 put up the pictures of the Lafayette piece. They were a great help in my explanation. Otherwise, I would have used the term "receiver in," rather than "source."
Thanks for the compliment, and for correcting me on the "Composer A" thing. That is why I always use a phrase like "as I recall" inviting a correction, if needed. I wasn't sure which one worked best for QS, having never sold this piece.
No, I didn't work for Lafayette. A local chain and Pacific Stereo were my work addresses in the '70's/early '80's. I bought my first Quad piece, Akai CR-80DSS from Lafayette. I bought lots of software from them and frequented their stores.
BTW: Lafayette first cropped up in both Chicago and Milwaukee in the early '70's. They successfully filled a gap left when Tandy bought and dismantled Allied Radio, who was king in our market until then. Hi-fi, CB, parts, tapes, records and virtually anything electronic were the benefits of both chains. And NAME BRAND gear, along with Lafayette (or Allied/Knight Kit) products made them much more enjoyable to shop than Radio Shack (or Tandy-ized Allied.) And I got to know several guys who worked for Lafayette, who impressed me as knowledgeable.
Your answer is very thorough ... One statement you made, however is incorrect: the Composer "A" circuit was used for the Sansui/QS system, and in most cases worked better than Sansui's own system (something Sansui wasn't too happy about). The Composer "B" position was for a less dramatic surround effect, but was in fact the basic matrix, or "SQ-M" circuitry. I found it utterly useless for anything...
...I believe you said you, like myself, are a former Lafayette employee...
Linda's explanation is quite thorough. If you we're looking to get into 4 channel sound, a common question was "can I keep and use my current stereo system or do I have to trade it and start over with a quad receiver?" These add on units were created to specifically address that need. The tape monitor circuit interrupt was a great way to marry the units together and provide a master volume.
In my experience, these "converters" didn't sell well. The lone exception was the Marantz 2440 which we blew out at $99 on a special purchase. Even I bought one. Complete Quad receivers sold far better.