I just went and looked, I could not find any part that looks like that. Sorry. .??
If you have any idea where it might have been let me know and I will go look again.
Maybe join the MX5 forum and post your pics there on the ND forum? A lot more users and more time with this roof. Just and idea. I also would be more inclined to drop by a Mazda dealer than a Fiat 124 dealer to see if they know what it is.