Either you have checked this personally,
or it's a guess.
Actually, it's an interesting observation. After all, it would be more cost effective, to produce 1 mainboard, and have 2 different cases, than to produce 2 mainboards, and 2 cases.
If I have a 500, I would be tempted to use a dremel tool, and remove the covering. After all, the sellers would probably not see this, if it were turned in for repair or replacement.
So in reality, unless somebody has tried this, we really don't know.
The only internal difference, would be the stock rom Acer installs. 1 for the 501, and 1 for the 500, which are identical except for 3g support.