Interestingly, the manual for the RB67 ProS states that the 127mm lens is the only one that will not have vignetting when the no. 1 and 2 tubes are stacked together, but I can't say that I've ever seen it in other lenses.
Thanks! And yes, that may be so, but the 127mm with two tubes is going to have some pretty bad image quality in the corners! More of a stop-gap solution in my opinion.