The old makers can be identified visually. Kenna sometimes (always?) put these lovely zigzag edges into his/their regulator keys - see the cover of the Drones and Chanters Vol. 1. Coyne and Egan usually made the regulator endcaps out of a single piece of ivory, although I believe there are a couple of exceptions to this. The shape is very distinctive with each:

This is an Egan set; note that the ring in the middle of the reg cap is right inbetween the two on the ends, and that the front of the cap is dead flat instead of being rounded. The “teardrop” regulator keys are another characteristic feature; Egan may have pioneered this, or the Moloney brothers. Also, note the gap in the wood between the tenor reg endcap and the base of the F# key.

This is Coyne. Note the two decorative rings in the endcaps instead of three, and that the end of the caps are rounded - the shape of the caps are reflected in the drone endmounts, also - the narrower regulator keys, forged in their own distinctive shape; and the lack of a ferrule on the small regulators at the stock end, a simple ivory mounting being used, instead of Egan’s mount - ferrule - mount. Also that the gap at the end of the small reg is much smaller than with Egan, who perhaps lengthened the bore here for tuning reasons.

This is Harrington. Note the very fancy three piece regulator end caps, with an ivory end socketed into a metal ferrule, which meets an ivory mount which is on the end of the regulator (I THINK! That how they’re put together, Peter?). Only the bass regulator’s endcap is intact in this picture. This is unique to Harrington; I wondered at first if the Ebay Willie Rowsome set wasn’t a “budget model” Harrington or something, but it’s just Willie imitating him, it seems. The shape of the endcaps is, again, reflected in the drone end mounts. The tuning rod’s mount is also much fancier than Coyne and Egan’s simple spheres. Very fancy stuff here. The standing piece of the drones is encased in a long ferrule inbetween two mounts, another feature unique to Harrington. I’m told that the face of the stock of Geoff Wooff’s Harrington set has a silver cover soldered to the stock ferrule, with holes drilled in it for the pipes (of course); I can’t tell from these pictures if this set has this feature, do other Harrington sets have these stock faceplates? Also note the square section guide block on the longer regulator keys. Coyne and Egan’s are rounded on the top.

Tommy Reck’s Kenna set. Note the zigzag regulator keys, and the sliding mounting on the bass regulator bar, which fits into a wedge shaped plate on the mainstock, allowing for the bar’s removability. All the old makers used this style of bass bar, I believe. The Taylors were the first makers to solder the bar to the stock plate itself, since they almost always used their distinctive “wrap-around” style of bass bar which didn’t need to be removed.
There are many more distinctive features to each of these pipemaker’s styles; this is just a sampling of what is most obvious in pictures.