Moorhead Laboratories produced this tubular Electron Relay starting in 1915 in several different forms until well after the US entered World WWI. It can be difficult to determine who made examples of these types of tubular vacuum valves but there are a few clues to keep in mind to help. The pinch in the pictured example is more fat than long and narrow as have been observed in Cunningham Audiotron's, for example. DeForest tubular Audion presses have a shape that can be a combination of each the Moorhead and Cunningham tubes. The insulation on the DeForest examples are usually rubber rather than cloth. The Moorhead example above has an ER stamped on the plate which makes an ID easy but this type of stamping is not consistent. This particular valve has a double filament but they were made with a single filament as well.
Taken from Joe's article.
Thanks to Joe Gruber for sending the images.
The thin glass tube envelope is mm in diameter and, excluding the base wires , is mm tall.
Reference: Joe Gruber. Type ER was first introduced in 1915. See also 1915 adverts.
Updated February 08, 2021.