Marsden Bay is the stop before the Marsden Grotto pub/hotel , which used to be the terminus for the old open top summer service from South Shields Market to Marsden Grotto which was operated with South Shields PTE 1218 ( now preserved in Queens silver Jubilee livery).
The actual terminus is still there at the entrance to the Grotto car park so I don't know why the 50 doesn't terminate there