28B with BSI or BS1

The stamp appears to read 28B with BSI or BS1 below, then a sideways E on the third row of lettering. It has a plastic "C" shaped piece attached to a synthetic (nylon?) string that goes up into the metal piece. Only a screw at the top holds this up and I was unable to success remove this from its original spot without damaging the window. I was not able to see a metal retainer clip inside the plastic piece where the sash pivot pins go but it does appear to just be tied to it somehow. All the other windows in the house appear to be the same so I don't think anyone altered it. I can find no identifying information on any of my windows, all single hung with tilting sash, white vinyl. I assume builder grade based on overall condition and the house was built around 1999. Almost everything we've upgraded has been the original hardware/appliance/plumbing/etc.