The needle plate pins for these machines were made from spring steel and were commonly broken. Only a few of the German 400 series machines used these pins and replacements are not available.
The following picture shows a with only 3 of the 4 sides of each pin intact.
Magnetic Pin Set – Game Changer
700 series Singer sewing machines use the same diameter pins as the 421G and 431G. On those machines, the needle plate is held on with a magnet that is embedded into the machine case.
I drilled an offset hole in 2mm thick by 10mm diameter neodymium magnets and created replacement stainless steel pins on a metal lathe.
The stainless pins are corrosion resistant, durable and non-magnetic so they will not stick to dressmaker pins.
The solution is simple and it holds extremely well.
Drilling the Magnets
Drilling the magnets is very difficult. I used a carbide glass bit and a lot of coolant to drill mine.
If there is sufficient demand, I would consider making pins for others and having magnets custom made with an offset hole to fit these older machines.
NOTE
When you put a needle plate pin in the hole on the right (closest to the stop motion wheel), you need to be careful not to place it too deeply or it will interfere with the feed dog bar.
Replacement pins need to be two different lengths and the short pin needs to go in the hole on the stop motion wheel side.
Always turn the machine by hand after making modifications to make sure nothing jams.