A quick look at Michel Spezial identifies 15 different plate flaws for this stamp, none of which are slanted numbers.
The closest is 19 II, "rechte Wertziffer gebrochen" (Right numeral broken)
I don't have a copy of this plate flaw so I can't tell if the break might be hidden under the cancellation in your stamp. Perhaps a comparison with a BPP expertized 19 II stamp would be in order to see if it "leans" too.
Shotgunning here -- maybe your stamp is a precursor to the plate failure creating the broken numeral. I'm not familiar enough with printing methods to know if this is even possible.
Dan
