That definitely seems to be an error, but there apparently have been cases of curved FAL mags existing, and apparently some guy filed down a Bren .308 mag to fit in the magwell of the FAL, to great success.
Cultist chan fell into a warp portal and transported back in time to hang out with kiddo emps
Now that I think about it? Has anyone in the 40k universe ever tried time-travel? You'd think with their kind of advanced science/pseudo magic they could pull it off.