Akai Remote Control Boxes varied over the years, and their numbering system was indeed as strange as the numbering system for the tape decks... or even stranger!
Several remotes existed, most were wired, some later ones were wireless.
The older remotes for sets like the X-345 used rectangular flat pins and plugged into the front of the set. Later sets like the X-330D used an RC-20 remote box.
Numerically earlier, but actually a bit later on in time, the GX-365D, GX-370D, GX-280D, and GX-280D-ss sets, and perhaps a few others, used a rear connecting RC-16 which had 11 large round pins on a round plug.
Quite a few years after, the GX-255, GX-266II, GX-400D, GX-630D, GX-650D, PRO-1000 etc. used an RC-17, or RC-18 ( or perhaps an RC-19 ) which also had the 11 large round pins and a large round plug. These are also wired remotes.
The RC-70R was a wireless two piece remote which which also worked with these sets, it also had the large round pins and a large round plug.
The later remotes that fit the GX-77, GX-646, and GX-747 use a smaller 8 pin DIN plug as found at the far end of the wired RC-21, and the wireless two piece: RC-90, RC-91, and RC-92.