Dan,
You have to take the top off to see the number. It's under the rear window and stamped into the metal on the edge that rests against the body. I forget whether it's on the left or right side. In your case, the number should be, I believe, "3890." Look for that number on the edge of the hood (if it's original), on the inside of the tonneau cover lid on the driver's side, and on the v-shaped body-color plate underneath the transmission.
And to answer your other question, there is no difference between a Roadster and a Coupe/Roadster aside from the very heavy and expensive hardtop.
Douglas Kim
New York, NY
280 SL #016220