I bought my 11 year old lefty boy a left hand Stag Arms/Stag-15L Model 2L Pre-Ban rifle last year and we have made several range trips with it since. The rifle came from
Stag Arms with an ARMS #40 BUIS. Their website shows they have changed from ARMS BUIS to a different maker:
Your lefty Stag will function identical to any AR variant, with the exception of ejecting the brass to the opposite side.
The A3 flat top picatinny rail allows you to remove any A3 carry handle sight and mount any number of optics along with a BUIS also.
The Stag Arms lefty upper has an ejection port cover identical to any other AR, just on the opposite side.
The factory Stag lower will have an ambi selector/saftey switch and there are now good quality after market ambi mag catch buttons available for the lower.
Great rifle that he really gets a charge out of shooting. Stag Arms builds a quality product IMHO.