They came in Eblocs
Even when I was running sniper schools using the M1C/Ds in the 70-80s we got match ammo in eblocs.
How ever, the Air Guard used M1s for competition, they got the same M-72 we used in boxes of 20.
There was a lot of ball issued in 5 round clips, but they were for the 1903/a3s that were used in WWII and Korea.
Even 7.62X51 came in 5 round clips. There was a charger that went over the M14 magazine or you could recharge your magazine while it was in the rifle via the M14s clip guide.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071