One is clearly marked Apple /// and the other is clearly marked Apple /// plus, is this normal?

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Yes. They're two different machines.

What is the main difference between the /// & ///+ other than the name plate and different layout of the keyboard?

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Apple-history.com lists some of the differences. The Plus was an attempt to overcome some of the quality problems the non-plus had. History counts it as too little, too late to save the line. A nice, quick read about the computer and its history is here:

http://siliconuser.com/?q=node/18