I remember reading in the book "Up & Down with the Rolling Stones" by Tony Sanchez (who worked for Keef for 10 yrs, keeping him supplied with "substances") about Keef having customs made. That was his reason for having them made, the "equal spacing" of the 5 strings across the neck.
I remember years ago trying to get stones songs down & knowing that I wasn't getting them right. Even when I realised it was open G tuning, songs like "Start Me Up" I just couldn't quite get right. When I found out he removed the low E string (D in open G) it was like BAM, all fell into place. I've kept an open G five string guitar ever since. Actually a banjo tuning. Cheers