Im not surprised there isn't a Sunderland, Penshaw, Harraton, Washington route prior to the mid 70's - only because The Galleries wouldn't have been built and the majority of the outlying villages/districts on the route wouldn't have been there either.
Im pretty sure Lambton was only finished off in the mid 80's too.

The 138 does look like the obvious candidate as a forerunner as far as Swallwell Bank.

Even between Wrekenton - Low Fell, there would have been nothing but fields until Harlow Green was constructed, so if there was anything, it would have gone down Beacon Lough bank.