The simplest marking gauge is obviously the single pin gauge and of course it doesn’t take much to adapt this to mark out mortises, just a little imagination. I have bought these gauges for 20 pence here in the UK on a regular basis. On the internet new and packaged marking gauges seem to sell between £4-10, higher in some cases. Sometimes I will see the same make for double the price and the same price as combination gauges so it seems practical to buy these rather than marking gauges only. That said, I like to have both, and of course buying at 20 pence means I have two dozen in my Penrhyn Castle workshop. The pins are most often hardened steel and will ruin a good file in a heartbeat if truly hard, which I have known frequently. Small diamond files work for reshaping but I find it best sometimes to simply replace the pin with a suitably sized piece of piano wire which is hard but can be filed. You can buy different gauges of piano wire inexpensively here.
In the earlier blog I mentioned boring the hole through the beam or stem from corner to corner so that the pin trails visibly as you mark the wood. This makes the gauge work exponentially better, but it only works on marking gauges.
Older gauges often have a pleasant patina that feels right in the hand and needs no restoration work. The worst damage is usually water and as many come from damp basements this can be a problem. If this is an issue, separate the beam from the stock and leave to dry away from excessive heat. and prevailing breezes. I usually forget them for a couple of weeks and get on with life. There is no point sanding or planing to soon as the likelihood is that shrinkage will make the beam fit the hole perfectly. Once the beam and stock are dry, plane them if necessary, taking a thousandth off to clean up and create smooth surfaces. Rounded beams can be sanded following the contour. I sometimes apply two or three coats of shellac followed by furniture wax applied with steel wool. This will make the tool feel smooth and it will make adjustment smooth too.
Pix to follow