Interesting concept. At least it can be used to whack the opponent on the head when you run out of ammo
Judging from the magazine button cutout (and lack of it at the bottom), the compressed spring takes up quite a bit of space...
According to the egun seller:
Langes Magazin für P 08. Kriegvertigung in Terrassa 1936.Sehr gut. Gestempelt" Parabellum 9 largo".
Long Magazine for P08. War time production from Terrassa, 1936. Very good. Stamped "Parabellum 9 largo".
Would make it a Spanish made magazine. Never heard of this one before.