That’s one of my favourite ROO’s . It’s hard to tell from the images (plus I’m far from an expert ) is that a JF or a cheaper rep?
In the case of your watch @C Master JF stands for “just fainted”. Don’t worry. The movement was good but a tad clogged with dirty grease. Also it whispered to me that it got wrist time once every 300 days due to the sheer number of watches the owner has.