GE UV-210 Prototype

UV-210 Prototype
While my identification is somewhat tentative, I believe this tube is a prototype version of the UV-210 tube as developed by General Electric for RCA. 

 Development of the UV-210 began in 1922, and it first came into use in 1924 as a replacement for the UV-202.  The UV-210 was superceded by the UX-210 in 1925.

This tube is totally unmarked, but it uses a phosphorus getter (obvious from the rainbow coloration), which would suggest the GE Nela Park plant, near Cleveland, OH as its origin.  They used the phosphorus getter there until sometime in 1924 at which point they switched over the familiar silver-colored magnesium getter exclusively. 

The unmarked short-pin Bakelite UV base (below, left) is nearly identical to bases on tubes sold under the RCA name made by GE, lacking only the molded-in RCA logo and the word "Radiotron" that appeared on production tubes (below, right), as well as type number markings on the sides.

Unmarked UV Base RCA Marked UV Base
The shape and size of the bulb is identical to that of the UV-202.  The internal structure is similar, as well.

Unfortunately, I don't have a production UV-210, but I located a picture of this tube on the net, and they appear to have been made with Bakelite bases, and had no evacuation tips on top.