This is a 15 needle / tine 8mm Garter Bar:
(The two 15 needle version (30 tine) is also available in this store.)
A garter bar is used on a knitting machine to remove the knitted fabric from the needles, turn it around, and replace the knitted fabric back on the needles. The ability to do this for an entire section of fabric (not just one needle) is necessary for garter stitches, and some lace patterns and cables. It can also be used if you have a need to temporarily remove a project from the machine.
The Bond company used to sell a garter bar for this purpose but it has not been available in a long time. This garter bar is based on the original Bond design and works the same way. It fits 8mm machines. Purchase the number of sections you need, depending on the number of needles in your typical project.
Any number of sections can be used. The sections fit together and should be secured together using a piece of 1/4” threaded rod with nuts and washers on the ends. Threaded rod is inexpensive and can be purchased at a hardware store. The rod should be cut to the length of the assembled garter bars, with enough room to add a nut and washer at each end. Note that this just needs to hold them together for convenience, it doesn’t need to be very tight. The garter bars are offered only in the "polished" materials so the yarn can slide more easily on and off the garter bar.