A version of these did find their way on to MZs, hence the link to an MZ parts seller. The parts list shows both slide choke & starter jet versions & the starter jet version appears almost identical to the BVF carb in operation. Item 48 further down the page appears to be the plunger with a swaged in sealing face that closes the starter jet via spring Item 50. The cable nipple entry can be seen on Item 48, so taut wire to open starter jet it appears to be.
If it follows BVF operation, then the open starter jet will only "draw fuel" when the thottle is closed.