Gamma sleeves are either the red paper sleeves or the blue, white, transparent plastic sleeve. Gamma changed address from Ejército Nacional 167 to Emerson 432 in 1976, according to Discogs. So, with this information, the red paper sleeves should go with pre-1976 singles and the plastic sleeve should go with post-1976 singles. I believe the “Paper Sleeve (early)” is the first since it has a shorter phone number than the one I call “Paper Sleeve (late)”