The Scott number is 029.
Michel Spezial lists only one perforation, 13 X 12 1/2.
The number in the selvage (margin) was used for inventory/accounting purposes. It may be interesting but adds no value to the stamp.
The stamp appears to be a CTO. It is a neat corner cancel and it looks like it might be "Schwerin (Meckl) 1 ag." This is a cancellation applied to reprints/CTOs.
The value of the stamp was covered in previous posts.
Dan
