There are several dials that were made under the old 25860 reference and the newer 26300 reference. There is a difference in the case construction and how it is assembled. I would be surprised if an F series is a 26300 and not in fact a 25860. Since the watch has no papers, the only way to know for sure is to have the watch inspected by an AP Service Center, where the case numbers and movement numbers can be looked up in the factory registry.