Is this German? 800 silver Cigarette Case with 3 marks

[benaequee]

This has a GM (country?), an 800 mark, and another hallmark (pictured below in a drawing I made).




Is this Art Deco or Victorian? I was told it was Art Deco becasue of the design and 800 mark. I hope I put this in the right area, I fugured the GM meant Germany, but I couldn't find anything on the site like it.

[Neruda]

I believe the double cross mark in a triangle indicates Chekoslovakia, c.1929 to 1940.

The number 5 indicates 80 percent pure silver.