No.4 Rifle magazine question

I recently acquired some early 1st, 2nd and 3rd variation magazine housings for some SMLE restoration work I'm doing and with the bunch of magazines I also received a No.4 Rifle magazine pictured below (It has an AH prefix 5 digit serial number and a DP stamp on it). What I would like to know from you guys is there any logical reason why the back rib of the magazine would be removed (perhaps related to its DP status?) as I would assume it is now next to useless for its intended purpose? Also, if anyone can identify what the pictured magazine spring comes from (I found it stuffed inside one of the 2nd variation mag housings) I would appreciate knowing what it is from as it has me stumped.

The rib is brazed on, so its probably just dropped off this particular magazine, or has been removed as part of someone's experimental gunsmithing. Lack of the rib would make the mag useless even in a DP rifle.