The sr9 mags were almost certainly made by Mec Gar or one of it's subsidiaries.
Mec gar makes arguably the best magazines in the world. When Sig stopped using mec gar to make their mags, there were reports on sig forums about factory mag malfunctions, which people were not experiencing with the mec gar after market mags they bought during the same period. Also, around that time, Mec gar redesigned the sig 226 mag so that it went from 15 rounds capacity to 18 rounds capacity as well as adding a super-slick and more durable coating. Essentially, they were setting the pace on sig mags.
Now it looks like Ruger has stopped contracting out to Mec gar because Mec gar doesn't usually undercut it's contract work with aftermarket offerings. This can be a great opportunity for the smart buyer to pick up what used to be $40 magazines for ~$25. Essentially you're getting next generation version of the same magazine by the same manufacturer for 60% of the cost.
Also, a $25 magazine makes the SR series much more competitive with Glock from a buyer's standpoint. Before you might have saved 100$ on your SR9 over a glock, and then sunk the saved money into magazines. Now you can save on the gun, and pay a fair price on the mags - it just makes it a more viable alternative.
I'm not trying to shill for MG, I'm actually excited for myself and other Ruger guys. Someone needs to get that sr9 mag and make sure it's not broken or something.