There was a period, somewhere between the X61 and the X88, when it was numbered X6. I have an old timetable leaflet somewhere that offered about 4 journeys each way, maybe timed to be operated with a single vehicle (or two) from the Northampton end.
I believe it got the chop about the same time Northamptonshire axed large amounts of subsidy, as the route doesn't seem to have done very well with the lack of houses between Silverstone and Brackley, and Brackley and Bicester (and then the frequent services from there to Oxford).
I may be totally wrong, but I thought the current 505 (Brackley - Bicester) was developer funded, with the large new estates built in Brackley.
Brackley currently seemed devoid of a bus map to check: Northamptonshire don't have anything for south of Northampton, and the Stagecoach Northamptonshire map is now five years old, according to the file name.