Strange way of indexing on a taper though
More likely "retaining" than "indexing" I suspect., rather like the Easychange holders.
Is the taper pressed in to the dovetail slide or retained with a screw ? Either way it should not be difficult to separate the two and put a parallel spigot in place of the taper – you could use it on anything then. I have something similar (but not as nicely made) with a parallel stub – much easier to set up for clocking a bore on the mill than a DTI arm lash-up.
Nigel B.