John Ferrell wrote:

Just a little speculation about the groove:
It may be that if the pulley is transporting something like mag tape
this groove is necessary to keep an air bubble from forming and
"flying" the tape.

It would not be difficult to round out the bottom of an existing
pulley with a lathe-like setup. I save old drill bits to make special
tools for that sort of thing.

You dont want the bottom of a pulley for square section belts to be
rounded, the drive comes from the sides of the belt. Like the vee belt
on a car alternator, when it wears enough for the belt to bottom out on
the pulley, it stops driving.

The slit is there so that the flat sides make correct contact

Ron(UK)