You will not find this info anywhere but it is for real, if the parking sign is altered then the officer cant give you a ticket !!
If for example the pay and display sign is from 9am t0 4pm and the 4pm has a sticker on it(is not visible) then you cant get a ticket!!
https://www.amazon.co.uk/Avery-Dennison-Zweckform-Letter-Stickers/dp/B000KJMBL2/ref=sr_1_13?ie=UTF8&qid=1471521064&sr=8-13&keywords=black+stickers