This kind of temporary plate was introduced in 1992. You can see a small box between the red "E" and the serial number: even if it's very small in the picture, you should read the word "scadenzai" that means "expiry" and then the exact date (day, month and year) of expiry is marked: their validity should last 30 days. As for Italian temporary plates the letter "E" stands for "Escursionisti Esteri".
Thanks to Alessandro Libanore for this very nice picture.