Here's another first for the project: lumping two buses together, even if they are buses that - for us - do not exist.
408 West Croydon to Guildford
The 1935 route 408 was a main southern trunk routes, linking Croydon, Epsom and Guildford. It was extended to run to Warlingham Park Hospital or Farleigh for summer 1937. Perhaps through traffic was scarce, as this ended that October with the route shortened to Croydon, though it returned to Warlingham in the 1940s.
The 408 continued unchanged, apart from changes at the Chelsham end, under London Country and subsequent privatisation, up until about 2000 when it was finally drastically altered, becoming an Epsom local route operating between Epsom and Cobham, with a school journey to Chipstead Valley.
This information comes from the excellent red-rf website, as does this picture.
The 409 route is even older: it began operating on 1 December 1924, when it assumed the identity of London General Country Services’ route S4, which dated from 3 June 1922. It took 2 hours and 58 minutes to run from West Croydon to Uckfield via Purley, Old Coulsdon, Caterham, Godstone, Blindley Heath, Lingfield, Felbridge, East Grinstead, Forest Row and Chelswood Gate. In 1933 it was withdrawn beyond Forest Row (which was now Southdown territory), and remained in that form for the next 46 years (although the 'up' terminus eventually moved to West Croydon Bus Station). On Sundays, there was no service between Godstone Bus Garage and Felbridge, so through journeys from Croydon to Forest Row could not be made on this day and Green Line was the alternative. On 26 October 1979 the 409 was withdrawn between East Grinstead and Forest Row. From 31 December 1981 bus journeys on the 409 operated only as far south as Lingfield (Stoats Nest Road). The route is operated by Metrobus for Surrey County Council.
I learned all this from the eplates website, from where I also borrowed the picture. We do enjoy other websites as obsessive as our own.
Possibly the most interesting thing is how the route numbers manage frequently to stay the same, or nearly the same, through changes of ownership and adjustments to the route. I used to work with an ex bus driver (I wonder whether conversations with David were among the sparks which ignited this whole bus adventure) and he talked about how routes were lengthened -or more likely cropped as traffic grew worse - without changing their numbers.
Sometimes, of course, the 'family' of a route gets additional digits to the number, like the 63 and the 363. And when in 2003 the 10 decided never to go to Archway, the part of its route from King's Cross onwards was given to a new bus number, the 390, which also covers the 10 route to Marble Arch, before heading off to Notting Hill Gate, leaving the 10 to trundle to Hammersmith.