Power Meters / Powermeters (SRM, Powertap, Quarq, Ergomo, Vector, Stages, power2max, P2M, 4iii, InPower, Cinch)

I thought stages made a replica nds arm. As opposed to just inserting their gubbins in an excisting arm. So making drive side versions to similar quality as DA would be abig ask.

As above, they retrofit them to existing arms that they buy direct from OEMs.

You could probably squeeze one on to a Hollowgram DS crank as is, but then you have the problem that it would hit the derailleur cage...