Hello phrenq,
That is actually the norm for Europa series notes' serial numbers starting with S or U. Those Italian (S) and French (U) notes are numbered in such a way that the 2nd and 3rd characters (a letter and a digit) represent (or reflect, to be precise) the sheet cell where the note is printed.
Think of a printing sheet as an Excel table or a game of Battleship or any rectangular
atlas grid.
So, on the same printing sheet, you will find S
R5abcdefgh
x on cell E5 (its printing code will be S022E5, if it was printed on the Italian

plate #022), and next to it you will find S
P5abcdefgh
y on cell E6 (printing code S022E6). Sheets are printed and stacked, than cut, and kept in bundles. Quite often, you will find bundles of SR5 followed or even interspersed with bundles of P5, for example.
You are very welcome to keep the curiousity, but please next time refrain from showing serial numbers almost in full (to prevent fake entries).