I think that's an unduly narrow description, Dan. The E-61 design included patented features, but the design is much more than a collection of patents. You wouldn't describe your favorite car design by listing its patents, either.HB wrote:The term "E61 group" is frequently misused. Some attribute the term to the saucer shaped grouphead, some attribute it to the thermosyphon design, others attribute it to the automated preinfusion. While the first two attributions are common embodiments of the implementation, only the latter attribute is part of the patent claim; see E61 Group Espresso Machine: Is its reputation justified? and Is there a purpose for the E61 middle brew lever position? for more details.
The short answer to "what makes an E61?" is "the alternately seating valves." The familiar grouphead shape and its thermodynamics are just the particulars of an example embodiment of the patent.
What we have today is a continuum of "E-61-ness," which starts with a 1961 Faema Eclipse and ends with whatever remotely related knockoff is shipping out of China today.