Plastic substitution numbers

[save-ferris]

Does anybody remember before the days of electronic substitution boards, the fourth official held up yellow plastic numbers to indicate which players were coming off and then which was to go on. I have a full set from the 90s from Sheff Wed in a leather pouch with the premier league logo on it. There are numbers 00-40 if i remember rightly. Just wondering how much these might be worth these days as a bit of a collectors item and a part of history. Any ideas would be greatly appreciated!