To me it looks like an ordinary lantern-style toolpost with an insert for round cutting tools. The half-round part is there to distribute the clamping forces, so as to not damage the split part of the holder. I have a similar one (and figured out its use myself in those dark, pre-WWW ages ), which is bored 4 mm. It takes those pre-shaped boring bars that came with my Wolf, Jahn & Co. lathe kit. The cross-hole has the correct diameter for the tommy-bar with which you unlock the x-slide or tighten the screw in the lantern. In this way you can turn the holder to adjust the tool height.
We had a discussion of such toolposts in another thread recently and I posted some pictures of mine. Here they are again: