Could M1 rifles be purchased by mail years ago? Just asking--I don't know.
Yeah, they could. I bought my first firearm (a Model 1200 shotgun) by mail.
I remember lots of ads for 1911 pistols and other surplus firearms, prepaid, delivered to your door. Lee Harvey Oswald bought his Carcano and a Model 10 SW by mail.
Surprisingly, you can still order a gun by mail. The CMP
will ship an M1 rifle to your door.