Ohhh I checked the old docs my bad. I have never seen em but it could be a mistake, or @bean will know. I wonder if it’s an error from transcribing the old docs to the new ones.
Well, in Nostradoomus' defence, the OP didn't say "Batwing24" — I'm confident it's just a leftover from the 9mm doc of MBP-2021's "Batwing".
The pot in question is the B1M "DECAY" ... the previous iteration of Batwing (2021) was spec'd to have the little black-trimmer-like 9mm pots, comme ça:
I don't think these little black-beauties are as robust as the metal shaft 9mm pots, and will build my 2021 Batwing up with FULL METAL JACKETs ... I mean, metal-shaft 9mm pots.
For the BATWING24's DECAY, I'm confident you can use a regular bog-standard B1M 16mm pot, jcpst.
PS: while I can't see why you need an insulated plastic shaft, Bourns PDB12 series ... 12mm wide, but 16mm leg-spacing, I think. If you/Bean insist. You can get'm at Digikey, probably Mouser, elsewhere...