Picco plugs will also fit Nova engines, but Nova plugs only fit Nova (based) engines.
I will try to explain.
The thread on a Nova engine goes deeper into the headbutton then on Sirio/Picco/GRP engines.
Therefor also the thread on a Nova plug goes further into the direction of the conical part.
Which means that you can't use a Nova plug in a Sirio/Picco/GRP, because the thread in the head button isn't deep enough, but using a Sirio/Picco/GRP plug in a Nova (based) engine is possible, because there is still some thread left....
thanks for the explanation.