1966 Hagstrom "world's fastest playing neck" guitar catalog, page 2. The Hagstrom 'Secret', was, of course, their patented "H" expander-stretcher truss rod that allowed for a thin neck with low action.
The H expander-stretcher The thinnest, fastest playing neck in the world... made possible only because of Hagstrom's patented "H" expander-stretcher
The Hagstrom neck ...for fast fingering action ...for smoother, easier playing along with the entire fingerboard ...for the lowest string action and for the only guitar & bass neck that stays in perfect alignment... You must let your fingers literally fly up and down this revolutionary neck and rhythmic miracle from Hagstrom
This page also describes the Hagstrom in-motion vibrato, micro-matic bridge, electro-sound pickups, and royal-tone strings, fitted to Hagstrom instruments at this time.