You must be talking about either an Eagle Arms M-15 or an Essential Arms J-15.

Eagle Arms became Armalite, and Armalite still makes "bargain level" AR15s and parts under the Eagle Arms name.

Essential Arms is a defunct manufacturer, whose equipment was bought by DPMS.  Also Bushmaster used to build AR15s on Essential Arms lowers before Bushmaster started making their own.

Both are commercial AR15 lower receivers, and all standard AR15 parts and accessories will fit on them.  In fact, Colt is the only manufacturer who has made AR15-style rifles with non-standard parts.

The Eagle Arms lower will be made from forged aluminum, which is considered better, as forged aluminum is stronger.  The Essential Arms is cast, and therefore somewhat less desireable, but lower receivers (unlike upper receivers) see very little stress, and should last several hundred thousand rounds.

-Troy