I don't think that Joe Beck "is under the impression that he invented the alto guitar" Here is an excerpt from an interview he gave;

"It's a tuning that I came up with back in the early sixties actually. I played a lot of Bossa Nova sessions and I wanted a tuning that allowed me to really be a Bass player so I tuned the guitar down but that really didn't work so I came up with this alternate tuning that I use and it allowed me to play melody with one finger and chords with three fingers and bass parts with my thumb. And it worked great but I had no reason after that to do it until I met Aly and heard her play the Alto flute and it just occurred to me that now would be a good time to try it so I had someone build me a guitar, Rick McCurdy as it happens, and he made me a beautiful guitar and so I started using it on the concert stage. Now Martin guitars make the Joe Beck Alto Guitar which is a really beautiful instrument if you have six grand to spare"