21 and 28 stems are identical apart from the cutout for the lock on the 28.

You won't find a part number for it as it's welded into the lower triple clamp and will need the weld cutting off before pressing out.

The best way to do this is to not turn off the weld, but to cut it down to a 30mm OD, and then press out the shaft. This way you get to keep an extra 5mm or so of stem for welding into the NC35 clamp.