This is an interesting combination that I recently ran across.
The cover is, apparently, an FDC though the cancel is rather difficult to make out and there is no special cancellation. I believe it is November 28, 1949.
The more interesting useage is the Cinderella issue on the reverse, just under the sender's return address. It appears to be a very nicely designed issue for the National Stamp Exhibition of 1950.
I am only guessing at this point, but there is a chance that this cinderella comes from a small sheet and I would be quite interested to see the full sheet if anyone is able to share a picture of it.

