Yikes!
Your inside picture should go into the "rot porn" thread.
My guess is that window has been out before and the PO who attacked it did a lousy job of repair, as it needs attention again. He did leave you some clues, like the nails on the inside indicate that he pried at the inside of the window frame until it broke, then made up the trim that is presently nailed on.
The outside screws indicate that he removed the bungs and screws the builder used, but didn't bother to re-attach with bungs over screws.
What I think the builder did, was to use a frame that came through to the fibreglass from the inside, was screwed on in a bedding of 5200, from the outside, then the glass was bedded in 5200, then the outside trim was attached over it all with more 5200, screwed in place while the bedding set up, bungs added, then the dressing of varnish.
The above is how my C&L fixed windows are built, which I found out when I had a cracked glass to replace. Yours have failed, so may have been a totally different method of construction.
Sliders are another issue altogether. C&L did a lousy job on sliders as did many of the builders of the 70s and 80s.