I spotted this at the edge of Stewart Park, I believe ‘CB’ stands for County Borough as Middlesbrough became one in 1889 and was extended in 1913, 1929 and 1932. This also co-incides with the Middlesbrough Rural District of which Marton was part being abolished in 1932.
1932 Middlesbrough Boundary Stone
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Definitely County Borough – a status held by Middlesbrough until Teesside County Borough came into being in the late 1960’s.
There is another example of one of these stone markers on The Grove at Marton Station end (on the other side of the road from the station carpark)
Cheers, i’ll look out for it next time i’m passing that way.