I knew I’d seen that marking somewhere... Here you go
http://oldswisswatches.com/Spares/complete_listing/AM_72.htm
It’s known as the Ebauches Trust mark... and once you know that it’s easy to find, along with a great reference site for makers marks that I’ve not seen before
https://www.thiemard.info/horlogerie/tools/poincons-listing.php?lang=en
And if my German isn’t misleading me, this page would suggest that the combination of marks you have with the AM in a triangle and the Trust mark was only used between 1936 and 39 (hopefully living in Munich you have a better command of the German language than I do)
https://watch-wiki.org/index.php?title=Ebauches_SA
I don’t know how you have identified the correct mainspring from Flume, except by measuring the old one? Often old watches have an incorrect spring fitted, so unless you have a positive identification of the calibre number, you can use the “reverse engineering” calculator on this page to check
http://www.vintagewatchstraps.com/mainsprings.php
Good luck finding a way to remove that broken screw. If you have access to a lathe you may try to drill it out... it may just catch on the drill and unscrew if you’re lucky!