(1 hour ago)Ex-conductor wrote I'm absolutely sure about that, from experience as a conductor in 1970 and from my copy of the 1968 and 1970 timetables.
Further information: Up to the mid-60s what is now the 308 was run jointly between Tynemouth and United, using different numbers. Tynemouth 5 ran from Newcastle to Whitley Bay, St, Mary's Island and United 8 ran the same route, continuing to Blyth. Both ran to a 20-minute frequency, giving a 10-minute frequency between Newcastle and Whitley Bay.
Around 1967, United decided to change to three-digit numbers for Northumberland to avoid confusion with their Durham services. For example there was a 56 Newcastle to Dudley circular and a 56 Newcastle to Bishop Auckland. Initially they started to use 408 for their 8 service and Tynemouth continued to use 5. By 1970, when I was on the buses, Tynemouth decided also to use 408.
In the early 70s the PTE (presumably along with Northumberland and Durham county councils) changed the numbers, most of which have been mentioned in this thread. It was at this time that the 408 became 308.
I distinctly remember the Tynemouth 408 interworking with Tynemouth 3. A 408 coming into Newcastle would change to a 3 to run along the Coast Road, down the newly-built Beach Road to Cullercoats where it terminated.
I hope that's useful.

Interesting history, never knew about that, if I had to be honest, knew they used to be single numbers and always assumed that it stayed the 8 until the change of the 308 so learnt something new tbh.  

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I don't know if anyone knows, but I'll hi-jack this but does anyone know why specific numbers were picked for certain area - especially the 5xx in South Tyneside.

You'd always thought they would've just went clockwise with Newcastle 0-99 and Sunderland 100-199 as they're the two core cities.

The only logical way I can think of is alphabetical order ie.
East Durham - 200
North Tyneside - 300
Northumberland - 400
South Tyneside - 500
Tynedale - 600
West Durham - 700

but surely it's not that?