I didn't think about it either, until getting into the MG47. The specific reason for installing the left rail first is so the bolt doesn't rotate during cycling tests; and to align it when you slip it in the rear. My ejector rail came with a flat; standard-length factory ejector rail.
Also, left rail goes in first (and NOT the ejector rail!) so you can 'unchamber' the rds. Otherwise you'd have to cut your ejection port right away. Found that one out the hard way too DUH so now you know I am certainly no expert