It means that it is a 455HT (455kHz +/- 3kHz) filter. The designation "IT" is a filter ....

This means that it is a filter 455HT (455kHz +/- 3kHz)
The mark "IT" - it is a filter 455IT (455kHz +/- 2kHz)
The mark "F" - it is a filter 455F (+/- 455kHz 6kHz)


Marked on the filter: "50HT"
means a filter 450HT (+/- 450kHz 3kHz)


The full specification you can found of the filters on the page in the description.