Well my house was not neo-Breton but a traditional longère and to be honest those screws look like something the previous owner has done as there were never any screws in windows like that and OH replaced hundreds over the years all over Bretagne. You can get the internal fitting of wooden small panes to make the outside look as though there were the traditional 6 panes, my neighbour did that and we should have when we replaced the windows back in 1989 and I did not want small panes, something the ABF picked up on when I put in a permis a few years ago for a garden wall and we promised to have them fitted but never did considering others closer to the church had big windows. Those screws could also be a crude fixing whenthe windows were installed to keep them in place if the walls underneath were damaged or the windows did not fit properly but end of the day, they are not normal fittings and a professional would have done a better job! As an afterthought removing them would leave a nasty mess so if it were me, I would try and screw them in a bit more flush and then disguise with the paste you put over nails and screws to disguise them ready for when re-varnishing or painting again (can’t think of the stuff’s name offhand but have used it previously)
Plastic wood filler.
Well good morning all…& what a windy & rainy morning in Cotes d’Armor!
I would like to thank the responders warmly for their input. We’re no closer to an answer unfortunately.
I will approach our only ‘neighbour’ to see if she knows. Other houses in this immediate area are different designs.
I did ask 3 local jobbing builders but they couldn’t help. I doubt they are as a result of a previous owners actions.
If I take them out there will be a hole, obviously, which can be filled. If I want screw them all the way into the frame I will need to remove the screw then drill the holes deeper. There is only about half of the drill length visible, the holes bottom out.
I’ll post any further info I obtain!
Cheers for now! Dave
If they’re evenly spaced in each window, I reckon (as @Stella did) they were to hold push on glazing bars to break up the single panes
That was my first thought, but the lower screws in the 'photo would seem to be a bit low for symmetry.
Ring the previous owners and ask, or ask a neighbour if you have any 
I’m now picturing them with Halloween then Christmas lights 
Those screws could have held in place a sheet of plexiglass, inside and out, as secondary insulation, easily removed and replaced for cleaning. Cheap amateurish rough & ready triple glazing?
Our windows had that, a frame with glass not Perspex and spotted holes in the frame that you used onto the screw and moved to the side 5mm into the slot, it wasn’t on the outside though it might have been shades on the outside.
Or perhaps it for a decorative window screen, like
https://www.etsy.com/de-en/listing/1266384984/iron-grille-window-grille-iron-rust-iron
Worth removing everything after the “rust-iron” (i.e. the “?” and everything after that) in that link.
I just copied it from the browser, and didn’t think too much about it.
Stella has hit the nail on the head. I’ve seen that on a house we wanted to buy