I thought those fast track vouchers are only valid with a stamp of an airline, date and flight on it.


No, I do not believe these need to contain detailed information. I've used blank vouchers many, many, many times without incident - both departing and arriving - some of which have been given to me by a family friend who works at the airport, while others have been received blank directly on the arriving aircraft.

As with all things here, there could be new "rules" and "enforcement" at any time so YMMV.