The red arrow points out the spit of land on the west side, which is the corridoor side for ‘down’ trains (ie those going “from”, rather than “to", London) approaching Oxford station. In this way, all standing passengers getting ready to alight at Oxford station a few minutes later, would have a very good chance to see in front of them on a discreetly-white background, the slogan “Archer Cowley of Oxford for Removals - Telephone 2397” or similar text.
The Railway line is carried high over the river Thames at this point, so the passengers would have had to look downwards to see this billboard. To my knowledge the advert was present at this location in the late 1940s and 1950s, but it may well have been there for decades before those dates. Sadly, it never occurred to me to ask my father about it, how long it had been there, was it there when he joined the firm in 1926, whose idea it was etc etc.