You might have wondered last time you went shopping for some SIG 556 parts and accessories why you found an assortment of disclaimers notifying you that the parts might be incompatible for the SG 550/551 series of weapons, or vice versa, despite these platforms looking very similar.
This compatibility divergence occurred because of the way SIG's presence has evolved in the United Sates. Namely, not all SIGs are created equal because some products found in the United States are designed and made by SIG Sauer Inc., which is headquartered in Newington, NH, and others by SIG Sauer GmbH, which is headquartered in Eckernforde, Germany.
SIG Sauer Inc. is actually the younger sister company to the more well-established SIG Sauer GmbH. SIG Sauer Inc. is principally distinguished by the fact it was created to specifically cater to American firearm enthusiasts. Initially, this resulted in the mere importation of SIG Sauer GmbH products into the United States through SIG Sauer Inc., but, as time passed, the company went into a different direction and began to forego importation and pursued the production of their own unique products instead.
In a little over a decade, SIG Sauer Inc.'s product line has evolved from simply containing Americanized versions of their European counterpart's firearms, such as the SIG 556 being made as an Americanized version of the SG 550/551, to the purely American designed and built SIG MPX successfully launching. However, despite these being different business operations and products, just to add a little more confusion to their corporate co-existence, some parts on SIG Sauer Inc. products are actually still being made to be interchangeable with certain SIG Sauer GmbH products despite these split paths (a full list of which would require an entirely different article)!
Overall, it is highly recommended that before you purchase any parts made for a SIG Sauer Inc. product and hope to apply them to a SIG Sauer GmbH product, or vice versa, do your homework first so you don't end up disappointed in discovering you have acquired non-interchangeable parts!