Reverb.com currently has a posting for an ultra-rare Moog Vanguard Theremin, serial number 6021. The theremin is offered for $6500.
Here's the full listing:
Vanguard Model Theremin, made by Robert Moog, c. 1960, serial # 6021, dark varnish finish, mahogany cabinet, fabric zipper cover case. Height is 21 in. (53.3 cm.), 18 in. (45.7 cm.) width, and 12 in. (30.5 cm.) deep. Very nice and all original except for a couple of small internal plastic brackets (still included though cracked). There is one sealed crack along the upper edge of one side of the cabinet, neatly repaired. This is a very rare find, a fantastic example of a Moog-built Theremin from the beginning of the 1960's-several years before the development of the earliest Moog synthesizer prototype. Excellent + Condition.
The Moog Vanguard was one of Bob Moog's early theremin models, beginning production in 1957. It featured a solid mahogany cabinet and built-in speaker. Here's a tour of Vanguard #6020 from YouTuber "The Guitologist":
Additional photos have been archived in this photo gallery.
Source: Reverb.com
