The stamps with the numbers 1 and 2 and TI in a circle have been a subject of speculations ever since the Bremen cards were introduced. (For example in this discussion, Feb. 2024)
Many cards were compared to find an underlying key, but in vain!
There seems to be no clarification in the e-guide either (but further DP documents with a variety of mysterious stamps and notes!) 
here or here.
A nice weekend to everybody!