Finding Replacement Parts for Your Telegraph Instruments
It is quite common to find telegraph instruments that are missing one or more parts, which is not surprising considering that many of the parts are adjustable by hand, so they are not secured to the instrument very tightly. In addition, sometimes parts are removed by the operator and discarded; for example, someone might have acquired an early landline key but they wanted to use it with an amateur radio station. Not needing the circuit closer, they just removed it and tossed it into their parts bin, unfortunately for the future collector who would acquire this key.
So where do you acquire replacement parts? Well, the simple answer is you have to scrounge for them ! Serious collectors spend years building up their supply of replacement parts, such as springs, screws, key knobs, etc. When you are at a hamfest, antique shop, or just browsing Ebay, don't pass over an ugly "junker" key because it might have some parts you might need for an on-going project or a future project not yet on your radar.
"Junker" keys are also a good source for knurled-head thumbscrews. As you might expect, most instruments have knurling styles very specific to the particular manufacturer, so they are not available at any hardware stores.
In some cases, you might come across an early rare instrument that has very unusual knurled thumbscrews, in which case even finding a "junker" key to scrounge for parts is next to impossible. In this case, you need to consider having a professional machinist make the parts for you. Be prepared to spend some money if you do this, because there is typically a "set-up" charge for any parts they make, and then there is also the question of whether they even have the proper knurling tool to make the parts. (Many of the early landline telegraph makers used a very fine knurl on their screw heads, which is not used much today). Take the parts you want to reproduce to the machinist and gets his opinion.