Crown with no markings on it?

[AlanChan]

Hey guys,
I presumed this would be the best place I could get rolex advice so here goes;

I have a load of Rolex crowns, most of which look standard to me after a regular eBay/Google search.
However, one of them has no markings other than the Rolex logo itself (no dots or dashes), it is bigger than the normal crowns. I was wondering if anybody could tell me what this is from?
Many thanks!