Italy Scott 755 Overprinted??

Hello fellow collectors. I stumbled on a Italy stamp that I have problem identifying. It is a regular Italy 1958 issue that has one of the stamps (Scott 755) overprinted in red inc with "5" over L15 (15 Lire). Scott 2007 does not mention anything about this issue being overprinted.
It would be great if anyone could shed some light on what that is and if it is valid OP and if it is cataloged anywhere.

Thanks in advance,

Paul

I could only speculate as to why there is this red '5' on the stamp. I can't believe it is a valid surcharge as there was no real reason for a change in rate in this time period. Also, and I could be wrong, of course, but I can't recall ever seeing a red surcharge on an Italian stamp.

Somebody got a rubber stamp set for Christmas .

Thanks guys! No surprises here... )