Anyone heard of this???

Must be a RABCO arm, which Harmon Kardon used on their linear tracking turntables from the 1970's. Heard one years ago, well built and solid HK decks but cannot recall too much about the arm. Regards, Richard.

Yes, it was a linear tracking arm. I saw a few on thorens TD125 turntables. I've never known anyone to own one. I had heard that when they were working they were awesome.

Your friend indeed pulled out a Rabco. The one you describe is most likely the later model and was sold separately as the model SL8 E. It was powered by a dry cell battery, which would electrically cue the arm down to the LP and back up. There were two very fine "whisker" leads on either side of the arm, at the very rear. When the arm would go out of alignment (much beyond the required 90 degrees), a post brushing one of these whiskers would engage another small motor, and a bead chain drive would move the arm back into correct alignment. Proper adjustment of this arm was very difficult. Not only was it necessary to have everything extraordinarily clean for it to function properly, the hidden battery losing power could cause damage to the LP. I used one of these arms on my reference turntable for nearly two years, and have information as to mods, upgrade battery (faster response of cue and correction), and counterbalance and head shell corrections. The bad news is there are literally dozens of arms now that will out perform this unit. If you want to resurrect it for fun, I could possibly offer help.

M & K (yes, the woofer folks) were modifying these arms and for several years it was Absolute Sound's state of the art reference. All of the linear tracking arms of today owe their heritage to Jacob Rabinowitz, the RAB in Rabco. Gee, I'm giving away my age by remembering this stuff!