I think it all depends on which ones are available these days.
From memory, all of the "biomed" ones could be switched between units.
Some of the early ones (Bio-Tek, Dale et al
... and then later, Fluke) were actually made by an OEM (whose name I cannot recall just now).
Apart from eBay, I'm not sure who (in the UK) can suppy a DPM
these days. Ultramedic