It's my understanding that 'Y' capacitors are used from L to E & from N to E, and 'X' capacitors are for across the mains. (As in X = 'A Cross').
I'm sure someone will put me right if I'm wrong, as I sometimes am.
See attached sketch.
As to the FB10, the makers data only shows a cap across the mains - the FB10 MK2 circuit doesn't, but as has been said, like most manufacturers, changes are made after the data is produced, so if parts look to be factory fitted original, it's as well to replace them. In this case, a class X, which is what Chris has ordered.