"Pay IPD the ten bucks for their good set of vent window locks and forget it forever. They're listed in the cataloque."
The sliding locks will keep your windows closed--but not without a performance price. I have them on my '69 144S after failing to keep the top pivots and/or latches on by glueing (despite a rigorous cleaning regimen and using both regular mirror adhesive or an industrial product sold to dealers--and trying other pieces of glass/hardware--they just wouldn't stay--I gave up). I successfully (that is--they would stay glued for years--not months or days on my previous 144--a '68).
The sliders will leave a gap in the rubber gasket causing a wind whistle and--they are clumsy to fiddle with while driving--both for opening and closing.
If anyone is interested, I have a collection of glass (clear and tinted) and latches (old and newer style) -- for cheap.
One other thing--someone mentioned in a post about glueing the top pivot while it's on the car--that pivot slips right out of the gasket and can be glued on the bench---mark proper spot carefully.-- Dave