Following up on this old thread with some hopefully clarifying information on these filters. I have been confused by this as well, and in the process of figuring out which filter fits in my rig, I accidentally bought the wrong one off of EBay, leading to a lot of online research and hopefully some answers.
As far as I can tell, XF refers to the actual filter device (not including the PCB board), while YF refers to the whole package of filter device plus the board (PCB w/ connectors) that it comes on… and the boards are specific to the radio.
XF-115C is the CW & RTTY filter device (without the board), with 500 Hz bandwidth, for the 455 kHz IF.
YF-115C is the XF-115C filter on a larger board (~70mm x 23mm, 8 connection pins), which fits in the Yaesu FT-1000MP, FT-1000MP Mark 5, FT-1000MP Mark 5 Field, FT-847 and FRG-100.
YF-122C is the XF-115C filter on a smaller board (~56mm x 14mm, 7 connection pins), which fits in the Yaesu FT-817, FT-817ND, FT-818ND, FT-857, FT-857D, FT-897, FT-897D, and FT-2000.
Note that I don’t have this from any official source, so would love to have someone who knows for sure chime in. I found most of this info by looking up accessories for each rig, and noting the similarities in naming and board sizes, so there may be some errors or omissions.
Hope that helps, -Chris KJ6WEG