The Corwins were a stage name for several different groups of musicians on the Gilmar record label. Gilmar records recorded sound alike cover records of the latest hits and then issued them on cheap 45 rpm records. The company also issued compact 33 rpm records and supposedly 12 inch albums as well,
although I have never ran across any of the 12 inch albums in all my years collecting records.
Update 11/17/12: I wish to inform you that Value Records issued albums for Gilmar (Which I assume Value might have just been a secondary label for Gilmar), and I have located a 12 inch album containing Gilmar material.