I can tell you that it isn’t from a B-25 for starters. The S-13 connection looks promising but I’m in doubt. The rear end of the nacelle seems to slope down quite steeply on that S-13 photo which isn’t present in the mystery part. The top line of the mystery nacelle seems to continue and extend over the flaps.

Edit: These provide some more clues


(images linked from http://www.aviodrome.nl)

There is a very pronounced vertical line just in front of the leading edge that the mystery nacelle doesn’t have. Based on that I’d say it is not the S-13.