This is something that I haven't seen on model steam belts, are they ever seen at the shows anyone attends?
Photo credit OLD TIME STEAM POWERED MACHINE SHOP
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I don't know about at steam shows, but back in the day they were not uncommon in machine shops and the like, as they provided an easy and automatic adjustment for belt stretch, and made belt length measurements a bit less critical.
Back in the the day when I was a leatherworker I used a skiiving knife to taper the ends of leather strips.
When I saw this, I immediately thought that a spokeshave might do the same job.
Sure enough:
https://youtu.be/xQG6MDZZKLA
I do have my grandfather spokeshave.