As for not looking hand made, some who have modified are skilled machinests and their work would reflect that. Some of course are not and are merely tinkerers or simply lacking the access to good machines.

As for the two speed operation, that sounds logical but by simply having to needle settings would not be enough to make a big difference. You would need to also alter the amount of air intake and additionally, if possible to increase/decrease the venturi opening. Much like when you kick in a secondary set (of bbls) on an automobile.

Is there any way of switching from one NV to the other? I suppose a valve could switch from one common tank to divert from one nipple to another...

Ideas?

Actually, it was initially quite simple.  I had a Fox .19 (circa about 1954-56) with two needle valves for primitive throttle control in the early R/C days.  You adjusted the high speed run for one needle valve and the low speed run for BOTH needle valves (a kind of R/C 4-cycle; very rich!!!).  It was a real balancing act.  High speed was not very good but low speed was not bad.  It didn't work well but, remember, it those days we were thrilled to have ANY KIND of throttle control.  The "controller valve" was similar to a fuel line pinch off common in those days in racing or free flight.  By the time someone had "engineered" a rotating valve to easily switch from high to low and back again, someone else had engineered a workable carb and the two needle valve system went into R/C history!  I doubt that the two needle throttle control was available on engines for more than a year or so.  You can often see that middle 1950's Fox .19 I had on EBay with a single needle and a blank space in the venturi for adding a needle.  Incidentally, that was an excellent .19.  I still have a single needle version I bought at the PX at Lackland in 1956.
regards,
bill marvel

Robert