Question About A First Day Label With German 1937 Stamp

Any idea what is this? Could it be listed in Michel German specialized catalog?

The stamp is listed (MiNr 646). The cover is commemorating the opening of the Post Museum in Hamburg on 8 April 1937.

Thank you. Of course, I am familiar with the Mi. number of this stamp, but didn't know is this an official issued cover by German Post. In fact, it is a sticker stuck on the envelope.

Here is how it looks without cover...

http://www.ebay.com/itm/162140732908

I think that it may not be entirely correct to say that this was officially issued by the German Post. Of course the stamp was, and the cancellation was approved by the German Post, but this is one of many thousands of special event covers that were being created all the time by special interest groups who asked the German Post for a special cancel for their event. If you go on ebay and search on German covers from the 20th century you will see countless examples of such special events for which the German Post used a special cancellation on ordinary stamps to help the sponsoring organization to raise funds.

I don't think the handstamps here have anything directly to do with the commemorative label.

The stamp has been cancelled with the regular Hamburg postmark on the museum's opening day.

The second undated handstamp is from the Nazi (NSDAP) party welfare office in Hamburg.