I bought my 300SDL in April '17. Today I put on the spare for the first time. I had to take it off again, the wheel wouldn't spin when partly tightened. I has 15 'holes' like the 4 wheels that work but there are subtle differences, the most important being that the shoulder on the inside starts to bulge out 3 inches in from the outer edge of the inner rim. The circular lines in the first pic are from contact with the caliper body when I was spinning it after slightly tightening it up.
With a bit more tightening, I couldn't turn it by hand at all. I examined the wheel that had been in that spot, the shoulder doesn't bulge out until close to 4 inches from the edge.
The tire's OK, not great, but the wheel is cherry. It's of no use to me, I want to sell it. I'd try to sell it as a spare but I don't know what vehicles it fits or what it's called.
Here is one of the other 4 wheels:
Re the flat spot, a small and odd accident at the mouth of an off ramp tossed a piece of wood, maybe 5"x6" into the offramp lane slightly below where the accident took place. I was looking at the accident and missed the wood in the road (darkness); missed seeing it that is. Ruined wheel and tire. Found a wheel for $20. Pick and Pull score. Lucky for me, it lost air slowly, I was able to make it home, woke up to find it flat. I discovered the spare didn't work from the comfort of my driveway.