I am about to upload a Mark III cover, but just want to make sure that the Mark III is regarded as the Master System here. I can see both sides to it being the same and separate. And I do realize we are not really big on Japanese hardware too.
Yes, hardware wise, the Mark III is the same as the Master System.
I think that systems with many name should have all the names for example when you specify system next to the search box. Another example: Genesis is known as Mega Drive in europe.