Thank you all for feedback.
I havn't had the opportunity to compare the green receiver with other examples before. I agree that the one shown in Siegers cat. is slightly different, but that's not from an actual cover I believe? The example at Christ-Stamps in the link from perf12 above https://www.christ-stamps.com/en/i_...773&row_nr=2
, shows two green receiver cancellation and the top inverted one shows even the left edge of the rubber stamp (before B) which also can be seen on my cover.
In K.B Spezial Band I there is also an example with the left edge visible. I don't know if it's ok to show a picture from the book here.
I would like to have an expert to look at the cover and eventually get a certificate, but I don't know where to ask. I have been in contact with Dieter Leder before in other matter, but I can't get in touch with him.