German stamps on with special cards and postmarks

These 5 post cards with stamps on them and with special cancellations were included in a batch of German stamps - but I do not know exactly their significance for German philatelist. I assume they were akin to US Frist Day Covers - but the cancellation dates are not consistent with the 1st day of issue of the stamp.

So if anyone is familiar with these - I am sure it would be interesting to learn more about them.

Thanks for looking