This started out (about 1975) to be a finger comb for a universal dovetailing jig. The sales guy at the tool store tolm me I could get any size guide bushing to suit my Makita & Hitachi & B & D routers. I set about making this comb from marine ply (I have since scavanged a piece of the back) & the comb size was 6mm. I've carted it around all those years thinking I would "use one day". Even today companies like Leigh dont make one that small.
For a number of years I overthought the dog scratcher looking at ways to spring load it and pressureize the up/down. I tried it manually & both our old dog & younger (about 6 y.o.) liked the scratching so I mounted it on an out door picnic table after routing a couple of slots for vertical adjustment. It became "just do it".
At first I didn't see them using it but noticed that the Jack Russell wasn't rolling around on his back on gravel & concrete. I checked the fixed scratcher which revealed hair=presto he WAS using it. I then saw the poodle x maltese using but every time I grabbed my camera she had finished/was finishing her scratchas in the pic.
If I make another I would try 8 or 12 mm & the precision of the first would not be necessary for a scratcher. It could be easily done on a router table or tablesaw or with hand tools. The fingers would not need to be so long either but are useful for vertical adjustment though dogs will arch if a little low.