Wow. I'm glad to see someone else here had this question, other than just me.
I THINK, one of these rails is a Troy Alpha, and the other is a Troy Bravo. but not sure...
The rounded smooth one (residing on my SR556E) has threaded holes. I THINK there is a way to add rails if you want, and WHERE you want. It is a much lighter rail according to TROY's website.
I'm still gathering information on this subject, as it seems, not many other individuals here, have much to go on yet.
EDIT: ADDED:
IF these ARE Troy rails, they have been modified for the piston system. Maybe a Ruger mod?
Also, maybe you can purchase the "other" rail from Ruger? I wouldn't mind having a quad...
Here is the thread in which I brought up the subject: http://www.rugerforum.com/phpBB3/viewtopic.php?f=4&t=167762
...AND Here is the post:
Does anyone know which TROY rail is used on the SR556 or the SR556E series. (one looks like an alpha rail, the other looks like a bravo.
And will http://store.troyind.com/Quick_Attach_Rail_Section_for_TRX_Extreme_p/srai-vkg-pxbt-00.htm
THOSE work on the SR556E series?