Scott,
For a couple of years in the late forties, probably 1948 and 1949, I worked at an ancient garage in Oxford Street Sydney during my school holidays. About the only "modern" piece of equipment was an earlier version of your plug service centre. I don't remember an abrasive cleaning ability - if it was there it was not working. It had separate holes for 10mm, 14 mm, 18mm, 7/8" and 1/2" NPT plugs.
Even in 1948 a lot of automotive parts were still in short supply or very expensive in the aftermath of WW2. The owner was a bit of a bower bird, and would never throw anything away if it could have any possible conceivable future use. Among other items he had accumulated were several buckets full of used spark plugs.
One of my first jobs was to clean these using a broken hacksaw blade with a taper ground to match the taper of the plug's insulator, blow them out, check the gap, and test them. They were then sorted into those that failed under 60 psi which were rejected, those that failed between 60 and 80 psi, those that failed between 80 and 100 psi, and 100psi or better. These were then sold for, I think sixpence for the low testing ones, then a shilling for the next bracket, then one shillings and sixpence for the "good" ones. There was no lack of buyers!
I had an aircraft plug someone had given me. On the tester it was still sparking happily at 150 psi, the shop compressor setting.
Since he was only paying me a pound ($2) a week it was probably a profitable exercise. Most of my schoolmates used to go to the big department stores wrapping Christmas parcels for about seven pounds a week, so I was making no fortune - I think I learned a lot more than they did though.
Frank.