Here's a nice example that turned up on Ebay in February 2015. Being offered at £1,475 by Nick's Guitars, West Yorkshire, England.
"Vintage 1969 Ampeg Dan Armstrong Plexi Bass features a clear acrylic polymer body. The Dan Armstrong Plexi Bass was first introduced at the Chicago NAMM Show in 1969. This bass has a very early serial number and does not have the later fitted toggle switch making it an early one as far as I can tell. In excellent original condition apart from replaced pic guard and non-original screw that secures the pickup."
"The Armstrong bass has unique electronics with a single pickup that was designed in collaboration with Bill Lawrence. There was a modification advertised in the 70's but this bass has not been altered. This bass has the original wiring. Pot codes date to 20th week 1969 and serial number on the neck appears to date to late 1969. Also included is the original hard case pictured."