The short story is three styles of bale from manufacturing 1941 -1985; an inclusion of the correct corresponding shell and chinstraps (and the history when proved) generally create the rareness or value of the M1. - D bale ( like this example = very rare!) , Fixed bale (The rectangular examples, like you have described = rarer) and finally swivel bale (1943-1985). 10 Euro is a good price for a genuine fixed bale helmet - if has original straps, a GREAT buy.
The history of this great helmet is far too much for me to write now, but as I said there is plenty here, and members like Jake above know endless amounts of info on these. So get acquainted with styles of the bales and seams, then heat stamps etc and you'll have a great advantage if proceeding in the hobby. Have fun!