Standards? The E-61 was originally created and patented by Faema. It was invented in 1961 which was a year of an eclipse, thus the name-- "E" for eclipse, and 61 for the year. When Faema ran into financial difficulties and was taken over by the Italian government, three of the executives took the patent for that brewhead design with them and started the Vibiemme company (V, B, M for the initials of the three executives). All other E-61 brewheads are made under license of that patent. Even there, numerous variations exist. The VBM E-61 weighs in at about 9 pounds which is reported to be about two pounds heavier than some competing E-61s.
So as far as a standard, I dont know. Any company is free to invent their own brewhead-- see the La Marzoccco GS-3 for example. I suppose the only lesson here is that if a machine states that it has an E-61 brewhead, it does not mean that it is comparable to other E-61 equipped machines.