As Far as the tube type Marantz Model 7 preamplifier, there is just the Model 7. The 7c denotes a model 7 that was purchased with the wood case, though it does not actually say 7c on the preamplifier itself. That is what I have come to learn. Anyway, there really is only a Marantz Model 7 (tube) and a Marantz Model 7T (transistors). C is for case, but, like I said, you will not actually find a Model 7 that has a 7c on the back. It was purely for ordering purposes only: with, or without the case.
Further, thought some may call it this or that, there only exists a Model 7 and 7T. I don't know about other designations, but I have seen the model 7/7T with a rack mount faceplate here or there, once or twice on eBay, so maybe those other designations have something to do with that. However, I don't these are any offical designation applied by, or endorsed by the original Marantz company. Those other designation could also have something to do with various modification that were done over the years by various people, Sid Smith, and others, etc., to the model 7.
