I have always been confused by 1974 La Pavoni models. Were both first and second generation La Pavoni machines made that year?
Both first generation machines and many second generation machines have two heating elements. Some seem to have a 200 watt and a 600 watt one, others a 200 watt and 800 watt one. The idea is that one switches both elements on to heat up the machine, and when the relief valve starts hissing, it is up to pressure, and then should switch to the low heating element position. That is what the numbers 1 and 2 stand for on later early models. Still later they used a general on off switch plus one that turns off the high wattage element to leave just the low wattage one on. It should gently hiss with just the low wattage one on. (Very early machines have a two position switch, which selects between one or both elements. These machines cannot be turned off, and must be disconnected from power when not in use. One should do that anyway!!!)
Some first generation machines can be fitted with a pressure gauge, many very early ones cannot. I do not think one is really needed.
Once the machine gets up to pressure and starts to hiss one can turn the high power element off with the switch. My technique with a 1964 machine is to prepare and load the portafilter whilst the machine is heating up. When it starts to hiss I raise the handle momentarily to release a bit of water, using a ramekin to catch the water, and then pump the handle up and down rapidly about four times, being careful not to release any more water. Then I turn off the second element, and then I attach the portafilter and make the shot. (This is NOT what you do with a second generation machine. The first generation machines are, in my opinion the best La Pavonis ever.)