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See "Form I-130, Petition for an Alien Relative" section on the USCIS Frankfurt page: https://www.uscis.gov/about-us/find-uscis-office/international-offices/germany-frankfurt-field-office
You can try asking the Frankfurt office directly as they are the ones that make that decision: https://www.uscis.gov/about-us/find-uscis-office/international-offices/contact-us-uscis-frankfurt-field-office

I second this. Call the USCIS office in Frankfurt up directly, explain the situation and ask if it will be accepted as proof of residency.
 
When I did DCF, I called the local office and they were glad to answer any questions I had. You could also try emailing but it might be slower.
 
I wanted to add, if you decide to call and do get answer, please update us on what they say! Best of luck.

Hi there! A couple days ago my original post here got *tons* of responses, while interesting and IMHO well intentioned by all, mainly went off on a tangent...
In a nutshell, from the replies, whether or not we refer to it as a "DCF" or simply filing with the USCIS international Office (in my case, Frankfurt) - the most important point is that in any case it seems this will be my best/fastest option. Much better than the lockbox thing, (if I understand correctly).
 
Ok, now that we have the background, here's my next question -- as I mentioned on my last post, we are now leaning towards the CR-1 instead of K1. In order to be able to file the I-130 for this, I must be able to prove at least 6 months permanent residency in Germany (per https://www.visajourney.com/content/dcf). I am not sure if I can claim this, can someone chime in if it sounds like I'll be able to?
 
I'll briefly explain - I have an apartment in Berlin and have filed at the local registration office and received an official "anmeldung" for the apartment (registration paper). As a US citizen, I currently also have a 6 month "job seeking" visa for Germany, but this will expire a month or two before we get married. Then I will officially become a "tourist" for a couple months, until I get married to my lovely Croatian fiancee, and then we believe we should be able to file for EU residency based on the fact that she is Croatian. The entire time throughout this, although our wedding will be in Croatia and I'll be spending perhaps at least a month there to prepare, do family stuff, etc, we can officially keep the apartment in Berlin.
 
Even though I can maintain this apartment residency in Berlin for over 6 months, I still will be keeping my "permanent" residence in the US, (in other words I'll still pay taxes as a resident of Illinois as well as Federal US taxes) based on that Illinois home address.
 
Based on the above, will I still be able to file the I-130 and begin the CR1 process at the USCIS in Frankfurt for the German residence? Really am not sure. If anyone can shed any light on what defines a 6 month residency in a country that would be great! Thanks again!!