Memel Double Overprint Identification Help (Or Help!)

Can anyone help me identify this stamp. I am so confused by Memel overprints. Or maybe refer me to a good reference? I have an envelope full of these and I am trying to figure them out. Seems like this is one is an airpost double overprint; postmark is Memel. And I think the number across the middle of the postmark is the feldpost where it was canceled? I see that double overprints might be more valuable - is this true?

Thanks!

That's a Memel airmail stamp. The Germans overprinted the French stamp with MEMEL / 60 Pfennig for use as a normal stamp, then a few months later, overprinted them FLUGPOST for use as an airmail stamp.

The numbers in the center of the postmarks are the date and time of cancellation – 13 August 1921 at 6-7 P.M. If it's genuine, Michel CV is €25-50, depending on the paper type.

Thank you. Greatly appreciated.