I don’t think we ever manufactured a fixed balance TBU; although we might have played around with some very early ones in 1972/3…. Come to think of it, it is possible that the TBU1 was manual balance.
Certainly the TBU2 was an automatic TBU, it worked by minimising sidetone during the moments when ‘transmit’ was taking place. It worked very well on local ends, but, as with all those analogue auto systems, they could never cope with the delays inherrent in digital transmission. The TBU2 had auto correction for both level and phase, and I believe this was where I started to use a partial ducking circuit.
It worked reasonably well, and was a competitor of the Studer unit.
In the early 80s I used a more sophisticated version of the TBU2 with very fast acting balance, in communications systems for money brokers. These multi-line ‘broadcast’ systems were very successful until everything went digital!
I’m afraid I don’t have any pictures of these old units.