Thanks for posting! It looks like you have a Series 390, but the S in your stamp certainly is puzzling. If you remove your sash from the window and weigh it, though, I would be able to tell you the best stamp option.
Hi,
As far as I can tell the weight is 4.2 oz. A small part of the spring is broken off, so the real unit could be just a bit more.
Thanks
The sash weight is 17.5 lbs. Hope that helps.
Thanks
Thanks for the information. It appears that you will need a S390-18 with the option 1750.