This is a single phase, mercury vapor, arc rectifier. I believe this is a GE Type 33655 street lighting rectifier. A tube looking like this with a paper label is shown in A Pictoral Album of Wireless and Radio by Greenwood. This tube was purchased in 1927 by W6AM.
Length = 14.74" Diameter = 3.00" (less arms)
Max voltage = 3000
Max current =