Paul,
To check for a possible replacement, as Bjorn mentions; check the tag on each of the belts. There may be a date stamp that would give some indication to whether the belt manufacturing date is reasonable for your car's production date or possibly some sets are off something else.

For example a "3 E 68" date would be the 3rd week of 1968. (The E is something else.) See: http://www.geocities.com/jims68fb/Interior.html