One problem with all European parlor guitars (almost) older than the 1950s is the distance between the poles of the tuning screws. Old tuning screws have shorter distances between the center of the outermost posts. Not only that, there are 4-5 different variations in the distance between the posts as well. In the 1950s, the distance of the American tuning screws between the posts became a de facto standard, which is now the only one used by all manufacturers.
Early Levin (1900-1905) has the American dimensions of the tuning screws, it also usually has finer models before the 1950s. Levin worked as a guitar maker in New York in the 1890s and certainly got a stock of tuning screws and had good contacts to buy new ones. Levin is an exception, other European manufacturers rarely have American tuning screws and modern dimensions.
Sometimes original tuning screws can be used, especially the older ones before about 1910 that have the post on the other side of the rack compared to modern tuning screws (the post with the knob closest to the upper saddle). Such tuning screws are ideal on a notched head as the string pulls the post against the rack. They may feel slack but work just fine when the strings are on. The older tuning screws (as in the picture) are more robust and better than those that came in the 1920s. Tuning screws from the 1800th century can be handmade and great! Generally speaking, new tuning screws are still tighter than old ones, even if they are in good condition. CNC machines and computers make the fit better now than at the turn of the century 😉
So here is a problem that must be solved if the original tuning screws are broken or you want good functioning tuning screws rather than the original (which is always coolest!). For the most part, some tuning screw is broken and then you have not so much choice. An advantage of modern dimensions between the posts is that it is easy to change to new and better tuning screws whenever you want.
To install new modern tuning screws, the old holes must be filled with new wood, at least the two outermost. In theory, the center hole may be left, but in practice you want to have complete freedom to place the new holes as it may be crowded on either side for the new posts.
Ideally, you should have wood with the same vein as the head, but regular plugs in the red book work so well that you can take the easy route. Red book is a very hard wood type and the metal post has a wide abutment surface (not like a string), so it works just fine. Drills out for 8 mm plugs, you need 6 pieces.
The most difficult step is to cut off the plugs without damaging the head. You want to cut straight with your head and you rarely succeed even with a Japanese saw and great caution. But since neck and head should be black lacquered play little miss does not matter.
Make sure to fill with thin super glue that rushes into the plug and any cracks in the head. Then use a knife and patience to take down the ends of the plugs even with the head (you do not quite succeed when cutting).
Golden Age tuning screws have a small washer at the post on the inside plate, you do not get perfect fit if you do not chamfer the hole on the outside. Uses the same tools used to chop the string holes, but a larger drill can also be used.
It was tight in the grooves, but it should be good to string on. Squeezes with forceps so you see that there is not a gap between the tuning screw and the head (as it would be if you were not bevelled by the holes so that the washer is not in the way).
Also plugs the old screw holes. Uses toothpicks in wood and a little tougher super glue. Failure to do so will always cause some holes to interfere when you tighten the screws in the new tuning screw.