Not sure when they stopped stamping them with the patent info and lubricate with graphite.
Personally I find it quite easy to tell oval from elliptical from just a straight on photo.
Here is a picture of all of the normal sizes that were made. From left to right 1*B, 2*B, 4*B, 5B, 6B

And here is a picture of an elliptical and an oval. The oval does not have the hard line corners so that is how to easily tell them apart.
