Reference the 30 round magazine “nub”
This is what it looks like next to a 15 with no “nub”
FWIW
I suggest folks NOT buy 30’s unless they are willing to fork over the cash for a hardback
IME aftermarket 30’s are more often than not trash
So by the time you buy 3-4 just to find one that is reliable, you could have just bought a hardback
USGI splitbacks are hit or miss as well usually due to being beat
And most of the USGI marketed 30’s are knockoffs
No one has faked hardbacks because they are so expensive/tricky to make, so you’re safe buying one even as a novice
Stick with quality 15’s
Or bite the bullet and buy a hardback 30