"Doing this from memory and not reference"

Me too! ;-)

"the 75bps is the "reverse channel" it employs different frequencies and is used for channel administration"

1200/75 is known as "Asymmetric Full-Duplex"
It was used in terminal applications:
1200bps for data coming from the host;
75bps for data going to the host.

Normally, the data going to the host originated from a human operator typing at a keyboard - so 75bps was quite adequate
(cf V.90, which gives up to 56k download but only 33k-odd upload).

The 75bps was (sometimes?) known as the "Back-Channel; (some) systems had the ability to swap the directions of the 1200 & 75bps channels - "link/line turnaround?"