I find there is a number 58 in the metal part shown in the photo below. The number is nothing similar to the serial number on the major parts. I checked photos of other K-31. There is even no number there.
http://s1083.photobucket.com/user/LiGuang/media/Swiss%20K-31/IMG_1270_zpse4409124.jpg.html?sort=3&o=0
http://s1083.photobucket.com/user/LiGuang/media/Swiss%20K-31/IMG_1270_zpse4409124.jpg.html?sort=3&o=0