Sash stop
It is same on the other Sash stop also.
Is this some kind of secondary securing method.
I have seen something like this in automotive repair. I believe it was some kind of bolt that breaks at a specific torque.
Thanks Mike
Hi there Mike.
Thank you for reaching out to us. I'm afraid it's hard to get a good look at your sash stop from this photo. Would you be able to remove it and show us a few photos? I can't promise we'll be able to help but that should help us confirm.
Mike
Your top sash guide reminds me of the 18-179. Have a look at the specifications and see how they compare with your old part.
I'm not sure if you need that bottom screw to install the top sash guide though. If you don't have to remove it, the better. You would need to remove the glass from the sash and take off the top rail off the window most likely to access those screws to remove them. If you can install the top sash guides and the window operates then it may be best to leave it well enough alone.