"The 300 was a stopping service that ran every hour, and took over the work of the old established R&W Cardiff-Gloucester service as far as Chepstow, while the 301 was a fast limited stop service which ran non-stop between the Bristol suburb of Westbury-on-Trym and Newport Bus Station."
An example of service 300 is seen here in Cardiff bus station. Vehicle GAX 6C, fleet number R665, was registered in 1965
RC169 (the person) comments that this vehicle will have crossed the "second" Severn bridge on many many occasions.
In 1978 Red and White was merged with Western Welsh to form National Welsh and the new company was privatised in 1988.
After four years of struggle National Welsh went bust, the only privatised ex NBC company to do so! Stagecoach bought a large chunk of the debris and, for a while, ran services as Stagecoach Red and White.
Route numbers 300 and 301 cleverly fitted in with the Bristol area scheme but also had a touch of National Express in due course, although never part of that network.
Bristol Omnibuses became privatised as Badgerline and later merged with expansionist Grampian to form First Bus.
Paul Davies can find no record of an X74 at the Bristol end (thanks for your efforts, Paul); so, either the GBBTT was wrong, or Red and White called it X74 and Bristol called it X14. After all a "1" looks a bit like a "7" through a dirty piece of glass! We eagerly await an explanatory comment.
fbb does not have an exact date for the change from 300 and 301 to X14 (possibly via X74?), the present-day number, but buses still run every hour to Newport (no longer any through service to Cardiff but with Stagecoach journeys continuing to Cwmbran, their main depot in the area) and there are still peak hour route variations, numbered X10 and X11 similar to that noted as "A" on the X74 table above.
Current information on this group of routes is available from many different sources; and it to these that we must address ourselves, starting tomorrow.
As might be expected, it's not a pretty sight!
Meanwhile, here is an X10 in a version of National Bus Company stripey coach livery, pictured about 30 years ago in Cardiff bus station.