Just found this.
The “Baby Snakes SG,” was Zappa’s main guitar for the latter part of the ’70s. The guitar is not actually a Gibson, but rather the creation of “a guy in Phoenix,” who made his way backstage and sold the guitar to Zappa for $500. Though its playing feel is a lot like a Gibson SG’s, closer inspection reveals such non-Gibson details as a 23rd fret and some nifty inlays and ornamental woodwork. Luthier/electronics maker Rex Bogue—the man who Zappa also tasked to bring Hendrix’s charred Miami Pop Festival Strat back to life—added various delights to this guitar, such as phase switches and an onboard preamp (Bogue passed away in 1996). Great googley moogley! Go to guitarplayer.com to see Dweezil Zappa run you through this guitar’s tones while plugged into his dad’s infamous Pignose amplifier.
Found this too
Then one day Frank acquired possibly the most famous guitar in his collection. While playing a show in Phoenix in the 70’s, “some guy” sneaked backstage and sold Frank a “Gibson” SG for $500. Though the guitar wasn’t a Gibson, it had the Gibson name on the headstock and from a distance, it looked like a normal SG. Closer inspection shows some non-standard Gibson features. The most visible and obvious would be the numerous knobs and switches put on the guitar. Other features included ornamental woodwork and special inlays. This is the guitar I know most about. The inlays consisted of smaller than usual Gibson dot inlays, including a Star inlay at the fifth fret and what appears to be eyes or something of similar design on the twelfth. The guitar also featured 23 frets, instead of 22, which the Gibson is known for. The addition of this fret pushed the neck pickup back a bit, giving it a more unique sound. The pickups were special made, but not much more is known. The guitar seems to have employed a vibrola-style tremolo at one point, but it was removed and a stop-bar tailpiece was put in its place. Some features of the electronics of the guitar include a Dan Armstrong Green Ringer circuit, phase switching, an onboard preamp for 18db boost on output, and maybe even coil-tapping switches. This guitar, along with the “inoffensive” Pignose amp (more on that below), was the rig used to record Over-nite Sensation and Apostrophe(‘) and also makes an appearance on the film “Baby Snakes”, hence the name “Baby Snakes SG”.