Meaning that nowadays three ways sockets ( Schucko/10 A/16 A) are common enough, but a Schuko (or a three way one) will take two "modules" whilst both the 10 A and the 16 A are narrower and take a single "module". (typically electric boxes in the wall are either a model 503 - three modules - or a 504 - four modules, but these latter oness can only be found in more recent houses), so until the use of 504's became more common, historicallty Schuko's were rare, as it was not possible to have in the same box a bipolar switch+fuse+Schuko (while it was possible to have bipolar switch+fuse+16A) which is/was the proper setup for high power sockets and with a single socket installed, it resulted in looking asymmetrical, see:
https://www.elettronew.com/blog/2017/05/11/presa-elettrica-t...
JFYI, recently came out a new kind of socket (not yet very common) that allows in two modules to insert either a Schuko or two 10 A (or 2x16 A or 1x10A and 1x16A):
https://www.archiproducts.com/newsletter/dossier/193843
The 10A one is actually compatible with the class 2 (earthless) Europlugs:
https://en.wikipedia.org/wiki/Europlug
Schuko's have larger pins, and it is not a good idea to jam them in a normal Italian 10A plug as the plastic cover of the plug might crack.