The "2020" under the barrel just shows the date run that that Pietta manufactured the barrel. Previous Pietta revolvers, within several date codes, have shown variously "2010" and "2016", among others, on the underside of the barrel. It is Pietta's way of keeping track of production barrel runs that they have in stock. I have a Pietta 1851 Navy .36 CM/2014 that is stamped "2010" on the underside of the barrel. It is no way indicative of the manufacture date of the revolver which is dictated by Italian law.
Starting with the year 2020, the new date codes are Arabic numericals within the rectangle. This is not a Pietta thing: Italian law dictates date codes and proof marks on assembled firearms. I suspect Uberti and Pedersoli guns will start showing up with the new series, I just have not seen any examples yet.
If you want to see how Pietta transitioned from pre-CNC revolvers to CNC manufacturing and the steps involved, you need to watch this. Pay attention to the early parts of the vid.
Regards,
Jim