.
The slot on the Q8 carts are there for all manufactures (not just the 858.. & 80(D)ss) that made Q8 capable play back decks to automatically switch the audio tracks to output 4CH (instead of stereo).
Akai 1800:
If you want to dub a 3 3/4 ips Q4 to the Q8 deck on the side of the 1800 SS just make sure the speed switch is set to high.
If you want to dub a 7 1/2 ips Q4 to the Q8 deck on the side, then you need the optional capstan, and again make sure the speed switch is set to high.
QQ Link from 2004:
Akai 1800 SS Quad Reel | QuadraphonicQuad
Blurb from Reel-Reel.com:
Akai 1800D-SS Tape Recorder | Reel-Reel.com (reel-reel.com)
DESCRIPTION
The 1800D-SS was a single motor, four-channel ‘Surround Sound’ deck with
three speeds and a built-in 8-track cartridge unit which was also a four-channel
example. Copies could be from four-channel open reel to four-channel 8-track.
Speed change was effected by use of the two-speed motor and a removable
capstan sleeve.
The unit featured
four separate input levels, four microphone inputs and two headphone outputs.
It could also be used as a quarter-track stereo deck.
The 1800D-SS featured
three heads in the reel to reel section, but in an unusual configuration.
It sported a conventional two-head arrangement of one erase and one combined
record/replay head but an extra full-track erase head was fitted for 4-channel
recording.
A tape selector switch for either normal or S.R.T. tapes was provided.