Mark IIb+ : what's the deal?

I recently bought a Mark IIb head which has a mod done to it: there's an extra gain knob in the rear, where slave out would be. It combines with the master on the front somehow. I just don't have a clear mental model of what the mod does or how it affects the tone overall... short of fiddling, which I've done plenty of.

Previous owner wasn't much help; didn't really know.
Amp has a sticker from Bendinelli Amplifier Services in Sebastopol, so I'm assuming this is a semi-standard FX Loop level mod (ie. IIB+)

Is there a manual or "official" semi-technical description out there of what the mod does? Thanks!

It becomes the Master volume of the amp. Think of the front panel Master knob as an "FX send" level and the rear panel knob as a "FX return". I used to run my Mark I front panel knob at around 4 that had this mod done to it..

I googled " Bendinelli Amplifier Services in Sebastopol" and I think the is the same Mike Bendinelli that worked for MESA Boogie.

This is probably the mod that he did with Mark IIB effect loops which improved the effect loop performance and the reverb. Even if you don't use the effect loop, the reverb sounds better than the stocked. This "new control" works like a Master Volume overall even though you have one on the front panel.