stamped on back. 27 5 B.S.I. OB

Thanks for posting. It's difficult to tell exactly what this is. It could either be the Series 380 or Series 385 depending on the size of the bottom shoe. That detail is the only difference. The Series 380 comes with the 15-005 while the Series 385 comes with the 15-004. You will need to measure your shoe to confirm.

As for the option, it could be the 2750 but it is always a good idea to weigh your sash, just in case.