Windows refurb question

[LoneHort]

Morning folks,
I have started a long overdue job of resealing my windows and I decided to start with a smaller one to get familiar with the process.
I have a question on my frames that I am sure others have , Salar 40 early 1970's. Can anyone tell me why the frames have a second inner groove? When I removed the frame there was just a rubber seal pushed around this groove and it was not glued in place. I am wondering is it just to "bulk out" the frame or was it originally designed for having a second glass to make it a double glazed type window?
Pictures showing the frame are here

https://photos.app.goo.gl/hNAm1RsioFusfzSN7

Thanks,
Micheal

 

[Graham376]

My Moody has similar window extrusion and I wonder if its either a condensation trap or, just for a stronger frame.

 

[ghostlymoron]

Good question. I refurbed one of my windows on my mirror offshore a while ago. It had the same type of frame and was easy (and successful) to do.

 

[Spirit (of Glenans)]

My theory is that they used an extrusion that could also be used for a sliding window.
Alternatively, it could have been intended to bulk out the frame, as you suggest, in order to allow for the thickness of the head-lining, or perhaps provide something to which to secure an internal trim around the window.