According to Accordionlinks.com:
"Fritz Rockstroh was a manufacturer of accordions from Georgenthal, Germany, founded in 1920, and still around in the 1960s. They produced the Ferroti brand."
If this is indeed the brand origin of your accordion, then I think you could assume that it is from the "mid 1900s."
As far as value, it would definetly depend in the quality of the istrument. Does everything work? Is it clean, no smells, case in good shape, etc.?
Here, in my part of the USA, in good condition this accordion would probably be worth $100 - $150. I have no idea of its value in Europe or wherever....
By the way, in the USA, all of these "free reed" instruments are called "accordions," with regional variations for type. Here, this type of (probably diatonic) button accordion would be called a "concertina."