Hi, I work in antiques.

The marking on the bottom are called hallmarks. The '006' you can see is actually '900'. This means it is 900/1000 parts silver. The other two marks flanking the 900 will most likely tell you who the manufacturer of the flask was, and from what city. However they are quite worn and it is hard to tell in this case.

Regardless, this would be worth several hundred dollars at least, if you ever tried to sell it.

Building upon /u/critically_analyse 's earlier answer, your flask is a bit of German kitsch although I don't know much about the actual authenticity of the flask from that period. For that you would have to consult with an antiques dealer who specializes in the period.

Patriotic kitsch was popular amongst the Victorians, who were the foremost producers of objects of this ilk. The Germans, who were intrinsically Anglophilic throughout the 19th century, quickly followed suit. There were several workshops that specialized in patriotic items whose breadth in scale could range from simple porcelain items to rather sophisticated bud vases that imitated artillery pieces. Your piece, however, is a specialized form of kitsch referred to as "trench art."

Trench art was a form of manufacturing that made due with ersatz materials available at the time. In my estimation, it probably took off around 1916 across the Continent although its true apogee was in the inter-war period stemming from 1919-1938. Trench art was somewhat of a dark, humorous take on violent objects that were repurposed for decorative use and virtually all of them have some sort of memorial or patriotic element. It could also, uniquely, be made by virtually anyone and tons of soldiers produced trench art as did private manufacturers. This is an example of a clock that is considered trench art.

Trench art has a fairly small following of collectors. If you're in no great need of money, I'd frankly keep the piece for sentimental reasons although I'm not sure that it would command much more than the silver melt cost. Again, I'm not an antiques dealer specializing in trench art or Second World War militaria so your best bet is to contact someone in the field for a full appraisal.

Edit: Last comment. Your hallmarks may be hard to read because they were intentionally destroyed. Several pieces of metal that were stamped with the Reichsadler were intentionally marred before they could be brought home as war booty. This would explain why the silver content mark is left blank while the others are hard to decipher.

Note to OP, I know others have said it, but do not attempt to clean this thing. Tarnish can be removed without rubbing via chemical processes, and if you or your descendants ever decide to sell it, any cleaning not done by a professional is going to probably damage it in some way.

There's a great website at http://www.925-1000.com/index.html that has compiled a huge list of world hallmarks, assay marks, and maker's and city marks throughout modern history. You may find your info there. Remember, German officers in the early 20th century traveled as far east as Moscow, as far south as Rwanda, and as far south and east as the USA, Argentina and Uruguay. Don't limit your search to German silversmiths. It's as likely that this is was originally built in Sheffield, England as is that it was made in Mexico City.

One thing to consider is that particularly among the officer corps, before the war German officers and other aristocracy were very likely to have traveled abroad as private citizens, particularly in western Europe, Great Britain and southern Europe, as well as anywhere Germany had colonies or embassies. The fact that they were, for the most part, the landed gentry with historical traditions of military service also means that things such as this may well have been handed down or modified with the swastika regalia after the original owner or an elder brother, father, or uncle carried it in WWI or between the wars. The lighter appears similar to WWI "trench lighters", but they were certainly not unique to the war. You might expand your search the web for "unusual antique hip flask", "whiskey flask", "silver trench lighter", "whiskey flask and cigarette lighter", "modified hip flask", and so on in various combinations.

As another commenter pointed out, the 600 is a 900, so flip it around several directions when comparing marks. Also, the mark on the right side of the "600" (in the second image) appears to be a double strike, so if you can use your imagination to figure out what the shape was before it was double struck, you might have better luck.

The other mark could be anything, but the bit with the four spikes radiating from it at the 1 o'clock position of the mark could well be a miniature SS death head (skull and crossbones), above some other four element mark.

Hope some of this helps.

r/whatisthisthing is probably more appropriate for finding information about something like this.

Thank you to everyone for all your replies and informative answers! I had no idea this would blow up so much. I don't have any intention right now to sell the Flask, I just wanted to see if there were ways I could find more information about it. Again, thank you again!

What is that bullet shaped thing attached to the flask? Is it a mini drinking cup?