I would guess the 1930s. I stumbled upon a pre-Hohner button box made by Koch, which Hohner absorbed in 1929, so my box is ca. 1920s. The guy who restored it made a video, see link #1. Link #2 might help you find more info.
The Hohner site says they have been putting a Serial Number on the back of their accordions since 1930. They have all the records.
The accordion has no numbers on the outside anywhere and I am not going to look inside. I will just assume it is pre-1930. It was a great find, it has a lot of patina and some wear from age. But, whomever had it, keep it in tune and it plays just fine.