Not rare at all here in the states! I served a 4 year apprenticeship in the mid 70s, and a butt gage was one of the tools we were required to purchase, and used it for many doors UNTIL electric routers and mortising templates came on the scene (mid 80s).
The gages were dirt cheap, relegated to the bottom of your tool box, since not really used much anymore, but since most of us were "frugal", we didn't toss them. They are easily found at fleamarkets, and are priced a bit dearly, because they look "complicated" to those not familiar with them.
I left the trade in 1989, and in the years since, have hired the contractor I used to work for, to do additions to my manufacturing plant. During those times, I've looked through the tool boxes of carpenters on the job. I was required to have a (very specific) Stanley 42X sawset, but no saw files! Today's carpenters carry a disposable hardpoint saw. I had to have a standard angle block plane and smoothing plane, but no jack or jointer. Today's carpenters "might" have a block plane, but they all had chisels, and a rather coarse carborundum stone.
The quality of their work, in our offices, was excellent, though. Their ages were 30s and early 40s, and I introduced a couple to sharpening-chisels, and plane blades (if they had a plane). I also suggested picking up a vintage saw or two, but interest was low, as the dispose-a-saws, were dirt cheap. I did loan them my Lion miter trimmer, which was used.
Back to the butt gage, they were very common here and used, until made obsolete by routers. Would I use one today? If I had one or two doors to hang, just a maybe!
