Vintage Synth Repair

Hello everybody, this is my first post.

Recently I moved to a house whose previous owner used to work in a
studio in the 80's, and after the business ended he got some studio
stuff which he eventually gave me.

One of them is an ARP 3620.

Right when I openned it, noticed a very weird plug, which I thought
was supposed to go on a DC adapter for power.

After much internet research I found the manuals, and from what I
understood, this is merely an "auxiliar" keyboard, to be plugged on an
ARP 2600 (hence the weird plug).

Am I correct? Since I don't have an ARP 2600, the 3620 is useless?

Thanks in advance,

max, from Brasil.