So, I emailed Johnson Controls as well and they answered me:
Good Morning Ulf, I can confirm your battery was manufactured on Sept 2015.
Kind regards Controls Power Solutions EMEA
So, I was right in my guessing and Johnson Controls (the manufacturer) probably has the same stamps for their batteries produced for VARTA and BOSCH. I followed this guide from an older StacExchange thread: https://mechanics.stackexchange.com/a/35523/40744
The key thing is to know what to look for. Batteries sold in the EU must be permanently marked with a means for manufacturers to identify production batches (if needed to recall a defective batch). This requirement does not extend outside the EU, but as many manufacturers have no idea where batches may end up being sold, most arrange for batteries to include the information either explicitly - often on a label stating clearly the year/month the item left the factory - or in a code which is sometimes buried in a lot of other numbers.
A genuine Bosch silver Battery has a very long number laser etched into the case - such as GBC1110330337 290A 24 Ignore everything on the printed labels. With batteries what matters is the number laser etched or physically melted as a string of numbers/letters into the case. To find the month of manufacture ignore the first three letters (GBC) it is the next 3 numbers which will identify when it left the factory. Then look up the three digit code here - http://aa-boschap-ru.resource.bosch.com/media/commonly_used_media/parts/repairs_and_service/_01072015.pdf
