Measure, in millimetres, the outside diameter of the threaded part, and the length of the screw, then go on to eBay and enter a search for x mm (diameter) by y mm (length) countersunk A4 (marine grade) stainless steel screws into the search. For example "4mm x 30mm countersunk A4 screws." Look for cross head screws to match those in your windows rather than slotted screws, order a sufficient quantity for the job plus a few spares, wait for delivery and then use them. Be aware that A4 screws will not tarnish as easily as A2 screws, but A4 screws are made from a softer material than A2 which means that if you have to use superhuman strength to tighten them into place, you are more likely to damage the heads of the A4 screws. Such damage can be remarkably easy to occur, particularly if your screwdriver is worn or is not held firmly in place or is not properly aligned with the screw. It is possible that the existing screws might be made from aluminium, though I don't really think that very likely and yours do look to me like stainless, but check first before you order. If you are ordering within the UK and are reluctant to use eBay, you could try Clipper Marine who are the distributors for the boats and their spare or replacement parts, but be prepared for high costs from them.
