My educated guess is that this Captain Marvel Jr. cover was by Mac Raboy, but I’m just not 100% certain.  I suppose it could also be by C.C. Beck, but I doubt it.  Regardless, good ol’ Freedy Freeman holds “The Key to the Mystery”!  Check out this model sheet by Raboy on how to draw the good Captain.  Comic Coverage also has an interesting interview with David Raboy (Mac’s son).