I am by no means an expert, but I believe the 'clamshell' top refers to the design of the control panels on the Imperial and Custom Imperial line of Frigidaire washers and dryers in the early 1960s. I don't have a picture to illustrate, but the panel was such that it looked like a partially opened clam shell like this: < The upper part, I believe was lighted and the lower part had the actual control panel dials.
Hope this helps!!