From what I've seen and been told, if the FDV is in the middle position of a factory 3X, it would be marked M. If you were to order an FDV from the factory, and told them it was for use in the middle position of a 3X, you would likely receive one marked B, as I was. If it was for the bridge, it would be marked N.
To my knowledge, Dingwall has not delivered a 3X with 3 Ptones. If you were to retrofit one, the PUPs from Dingwall would most likely be marked N/B/N. However, if you were to order 3 for use in a 3X, maybe they would mark them N/M/B, I got 3 Ptones and I FDV to use interchangeably between my SJ5XXX and SPJ5, and the tech and sales guys knew that when they shipped them to me.
As per Sheldon, I am the first owner who is going to try a 3X with FD3/Ptone/Ptone, or an SPJ5 with P/Ptone or P/FDV.
I am only experimenting this way because no one else has shared if they've done it. I am trying to get a better solo tone from a bridge solo'd setting, or in the case of my 3X, a stronger middle PUP solo'd, if I keep it wired this way.
Hopefully, this makes sense to someone besides me?