WHAT DOES 3RD ONLINE MEAN?

† NOTE: The receiver-mounted, Picatinny-design rail is intended to accommodate most optics and optic mounts meant for use with Picatinny/ MIL-STD 1913 rails. However, on account of lessened clearance between the highest of your receiver and The underside on the optics rail, optics and optic mounts that extend a lot more than .Make sure you Notice