Almost all the negative carriers for 35mm film on Leitz enlargers follow the pattern of a single metal plate with few variations. Yes, Esmiralda, I know there are 3x4 and 4x4 versions which I have, with glass, never used and am never likely to.
However to start to plough a new furrow I wonder just how these items were made? I ask from the practical viewpoint of a man trying to replace the red gelatin in two standard holders. At a guess the channel is formed by soldering shim onto the base brass after the red windows are filled. Trouble is you just can't see the join. ( Avoiding obvious humour)
Use of a magnifying lens on the outer edge shows no trace of two parts having been joined but a faint line is visible around the film aperture, always assuming you have not opened it out to 25x37 with a file. The tiny screws holding the silver 'ramps' are not involved.
I have found a holder from a pre Valoy model which must date to the 1920's which had missing red gelatine but an obvious milled recess into which a replacement gel can be fitted in a few moments. I used a slip of the safelight that used to come in Ilford MG filter sets and bedded it in with UHU which did not react. However, no nearer the real question of how do we repair standard negative carriers?
The only rather poor solution I can come up with is to cut a mask such as appeared in the Morgan Leica manual at one time.This was a possible cure for Newtons Rings. Made from red acetate and covering both the gaps it should then show film numbers against red film.
Yet again, any ideas welcome. This is a picture of the older carrier which was easy to repair.