A319 Captain reported a deadheading Flight Attendant reported to them the placards for opening the main cabin doors were incorrect. The Flight Attendant did not notify the Captain in a timely manner.

Narrative

Uneventful Qual Line Check flight to ZZZ from ZZZ1. After the passengers had deplaned; our Flight Attendant (FA) cabin jump seater entered the flight deck and informed the pilots that doors 1L and 1R had incorrect placards installed. Specifically; the placards with the word 'OPEN' and the illustration on how to open the main cabin door in a priority situation. The placards for the 'Left' doors are mirror images of the placards for the 'Right' doors. The difference being the orientation of the diagram. She explained that she had found this discrepancy on 'at least 10' aircraft; that it was a 'no-go item' and that we should write it up once we were inbound to ZZZ1; since cabin inspectors would be looking for this issue. I thanked her for bringing this to our attention; and explained the requirement to write up any known discrepancies or irregularities as soon as they were known. We contacted Maintenance and described the issue; and they dispatched a contract mechanic; who did a fantastic job deferring the issue. They printed facsimile placards and fastened them in place; resulting in minimal delays. I bring this to the attention for several reasons. This aircraft as obviously operated numerous flight segments with improper placards installed. I recommend reaching out to Maintenance to see if a fleet directive or other inspection program should be initiated to check for similar errors. Should consider reaching out to the safety team in case there are mechanics who should file reports for the incorrect placard installation. Should consider reaching out so that the jump seating Flight Attendant can submit a report with her perspective on the event. What is most troubling for me is that she likely noticed this issue during boarding in ZZZ1; as she was one of the first people on the aircraft and had notified numerous crews of this issue yet she chose to wait until her commuter flight arrived in ZZZ to notify the crew of the discrepancy. This act indicates that our Inflight Services partners might need to re-emphasize the importance of notifying the flight deck as soon as they find a defect or discrepancy with the aircraft.