Don't know much about them, and can't find my catalogs from that period, but I have a Sescom RS-SB dual balanced-unbalance box that has a SIP-1 in it. I think it says copyright 1991.
I was given the RS-SB and was looking for info on it some years ago. I recall seeing an old catalog with some modules listed, but I think the modules were only around for a short time.