The frame and wheel assembly appear to be that of a Coffman. The label on this trailer says it is a Sporty, yet the person who owned it advertised it as and Alder. The snaps around the trailer, show that a cover was a part of the trailer. Unable to tell if the frame and the body are steel or aluminum. If you have any info on Alder trailer, please tell more.
Found this ad on The Samba. “One wheel trailer 1961 ALDER (60 lbs). It is a 1961 Alder Title says it 60 lbs. It is starting to corrode in two small spots on the right lower middle side if you look you can see it. I have a vinyl snap on top, sits up about a foot higher needs to be repaired (if I can find it).”