Found along Lyme Regis Coast.
Difficult to tell, if you wet the underside and take a picture it should be easier to see. It sounds silly but it will make it stand out more if its shell, ammonite or just the mineral calcite. Same sort of picture as the forth pic only wet.
Probably just a stone, Lurty.
It is elongate just in reflection of the layering (stronger one way than the other), and round somewhat by chance but influenced by having similar strength in all directions along the layering.
One could argue it looks like an operculum, but I think that may just be coincidence.
Ammonites had a thing called an aptychus, whose purpose is open to debate. It may have been an operculum or could have been a mouth part.
If you are able to bring or post the specimen to the AMC centre at the museum our Earth Science officer would be more than happy to look at it for you to see if they can help solve the stone or fossil mystery.
Thanks everyone, maybe the option is to post it in as its very small. Or I might just book a time and travel down with everything I would like to get identifed