These are an excellent, must have braces (or suspenders for you Americans) for anyone reenacting the Finnish military of the 1940’s.
The button-on braces were the norm for a soldier of the 1920’s, 30’s and 40’s. Back in the day the military trousers were not even designed to be worn with a belt. Instead, you used them with braces. While there was a standard Finnish Army-issue type, the army issued mostly various types and styles of civilian braces. These ones represent one of the most commonly seen types of suspenders out there.
Helsingin Olkain Oy makes these suspenders. A traditional company with history starting in the 1930’s. These suspenders are hand made in Finland in the traditional Y-style and leather details. The adjustment clasp is completely made out of metal, with no plastic in them. Can’t get more accurate than that.
The size is the maximum length of the suspender. To measure your size roughly, put on your trousers and measure the length from back button to the front button over your shoulder.