For me, the single most important development in espresso was the invention of the spring lever in the form of achille gaggia’s machine in 1947. For the very first time in history, true espresso as we know it was created whereby 9 bar and greater pressures were exerted for the first time with declining pressure profile thereby creating the crema containing cup of concentrated coffee flavours we love today. (Interestingly, I understand from hedrick’s museum visit video that the first spring levers had double boilers and if so then double boilers date back to the genesis of the espresso machine…)
There was an aberration in 1961 when the pump machine was invented and then took over the espresso world, but thankfully the lever has always been held as the standardin naples and has made a comeback in the rest of the world in more recent years.