I found on a sales site this cutout of an entire postal (or administrativ?)
I think it's Mi # 52, Sudetenland-Rumburg, 1938.
Michel mentions vertical or horizontal pairs, with a decorative field between stamps.
On the left and right side there are parts from other 2 stamps.
I don't think the marking is postal (administrative ??) and I don't have any experience about overprinting (forgerie / genuine)
Thanks for any information about it.