This is something that's always been done (no date on ticket) by ball clubs.
There are many reasons for doing this:
Tickets to seats other than the most expensive box seats frequently didn't have dates printed on them, only game numbers. Your ticket does not have a game number, BTW.
Overflow crowds where you did not print enough tickets. This is a cost saving measure. Baseball franchises frequently were financially unstable before attendances grew to where they are today. If you print too many tickets and you have to throw them away it creates wasted revenue.
An event is postponed, creating an event date that wasn't part of the original schedule. Frequently they issued what were called "Emergency Tickets"
A tie in the standings at the end of the year that required an additional game be played. Tickets don't typically exist for this scenario.
I hope this helps!
