.. wouldn't you want a little drag on the reel on the left ....
there was planty of "drag" in my "prototype" heh heh, you can hear it too on the video.
I'd say, the whole point here is to somehow succefully wind tape to a 7", then you wind it from that 7" to another and/or later even play-mode-wind for perfection.
This "method" seems to be useful if you only have/use 7" machine(s) and get some pancake tapes for cheap

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Silly, thing, I was facing similar situation few weeks ago. I have couple 10" reel machines but I sold adaptors while ago, because I happen never use 10" reels.
Then I've got few 10" reels and some pancakes, so I wanted to get the tapes. This "idea" would work, but then I did not think of this and the first thing came to my mind was to make the damn adaptors. So I did, and the DIY-adaptors worked pretty well. It was a piece of work and pain in the neck to make, I must say. I actually made another pair later. See picture attached.
(I have bunch of detailed photos about how to make those, maybe someday I'll find time to put it all together for demonstration for DIY-heads.)