Interesting. I don’t think I have ever heard of any Thomas made out of some material different to regular ones. Be aware that how it feels, especially when you’ve just unpacked it, and the darkness of the sticker depend on how and where it’s been stored or displayed. Give it a week and see what it feels like then.
Some sharp close-up pictures of the undersides and their markings would be useful, especially if you told us which was which. Also, have you detected any differences between the wagons?
