If it is lathe it’s not terribly clear exactly how it fixes to the cross slide. T slot is unusual on a British low end machine. Looks as if one side of the slot has been got at too.
Strange.
Clive
there is a partial groove inside and the hex bolt probably served to lock and hold it onto a pin albeit of large diameter.
and yes I wondered myself if somebody had machined that slot at some point or that was just some peculiar feature of the design.
it’s still better made than most modern junk.