Martin GT-75 

An electric Martin guitar.

Free truss rod nut but excessive back bow in the neck.

Martin supposedly made 750 GT-75s (double cutaway) and 453 GT-70s (single cutaway) and 750 GT-75s, from 1966 to October 1968 (the two years they were in production). These guitars came in black, a maroon/burgundy and the red seen on this example. Also rumored is one left-handed single cutaway GT-70 in red, which I guess might have been a one-off prototype. These GT models evolved from Martin's F series, with special DeArmond pickups, F-holes and a laminated maple body. They were also the first Martins to use an adjustable truss rod (see below). This one appears to be an early 1966 from the serial number.

Cool Grover tuners and clean.

Neck screw cover.

Neck seems to have never been off. Looks like corrosion on those lower two screws but all four look factory and unmessed with.

Notes on Binding repair here.

Bigsby bridge - probably to be replaced with something adjustable for intonation/setup.

Original bridge (Bigsby?) from the factory seems low for the intended use. 

The metal Bigsby bridge someone had the need to shored-up with wood, probably from the excessive back-bow of the neck but still an unknown.