It's a common problem with those mags (Ramline & older Eagle mags), and there's no easy fix.  These were made back in the days when they cost $9.99, and weren't expected to last very long.  And they don't.

You might try drilling out the nub and epoxying in a piece of metal rod.  That may not last long either, but once it's broken, there's little harm in trying.

In the future, stick to mags with metal parts, even though they are expensive.  Butler Creek's "Steel Lips", Condors, and the Mitchel 50-round teardrop drums all have metal parts.  Next best are the Butler Creek "Hot Lips" and the new-style Eagles, which use a different type of plastic for the major wear parts.  The worst are the Ramline and old-style Eagles, which use the same brittle plastic for the feedlips and the locking nubs.

-Troy

There are no "red" parts in a Ramline mag.  Unlike the Butler Creek mags which use different (red) plastic for the high-use parts, the Ramlines are all one-piece.

-Troy