I was clearing out the cabinet under my lathe and came across 3 of these types of tool holder that have come with past machine tool acquisitions. I have never used them, have never wanted to use them nor do I ever expect to use them so they were transported to the "awaiting recycling" bin but en route they stopped off for a family photo.
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The centre example is very well made and is obviously for holding boring bars, up to about 25mm. The right side one also appears to be for boring bars but is nowhere near the quality of the other.
I think that these types of tool holder were more popular in the US compared to rest of world.