I don't think I've seen this before, But
I was talking to C. Derrington back in about 1990 (yr.?). I was checking out some of the new F5L's he had for sale. I hummed into it to see what the resonant frequency was. I had just discovered this probably. He heard me do that and said that typically they were at Eb, BUT that Monroe's mandolin resonated at "E". Can anyone here confirm this? Or do you know if the Monroe models had this feature? Or if, anyone else ever tried this on a mandolin?