There were both pre-ban and post-ban Sendra lowers, and despite the old FAQ, there's no way to tell the difference between the two.  Sendra didn't sell complete guns, they just made lowers (though they did also sell surplus uppers; they just didn't sell them together).  And Sendra hasn't existed in about 8 years; the owner went to jail and some family members took over the parts business, as MA Parts and Model 1 Sales.  They have no Sendra records.

Given all the above, if this lower doesn't have some kind of written proof of its pre-ban status, you should consider it to be post-ban.

-Troy