They are called "silverface" due to the brushed aluminum look of the faceplate. Do a quick look on ebay, eliminate the ridiculously high opening bids that no one else has bid on, (which in case of the search I just did is well over half of them) and you'll find that in most cases you can expect to pay about twice as much for a Blackface than a Silverface, and at least three or four times as much for a tweed.(5 or 6 times as much in the case of a tweed Champ vs a silverface one) Face it, tweeds are actually museum artifacts, and if you find any genuine tweed Fender for under a grand, buy it and consider yourself extremely lucky, and I'm not going to compare Silverface equipment to tweed.
I suppose I could have ripped off some photos from other websites, but that's not my style.
If you need to see them, Google the Fender Field Guide. Quite simply, a Silverface Fender Amp is an amplifier made by the Fender Musical Instruments Company from late 1967 to 1983. I suppose that if you compare Silverface equipment to its earlier tweed and blackface brethren, the silverface amps are in many cases not that great, and that is the justification that has been used for a couple of decades by Silverface critics.
I'm also not going to get too deeply into technical details of circuitry.
I apologize in advance if I got a date wrong, a model wrong, etc.
There were a very few cosmetic changes made between 19.