I don't know what this sign means.
Its a fixed distant signal.
On 28 February 1975 an accident occurred at Moorgate, which at the time was on the London Underground's Northern City Line. A loaded passenger train ran at speed through the buffer stop and into the tunnel end wall. As a result, British Rail decided that in future all colour light signals reading into terminal platform lines would show a yellow rather than a green aspect when the line was clear to the buffer stop
In the early 1980s, a 'low cost signalling committee' was formed, tasked with finding cheaper means of signalling lightly-used lines. One thing that came out of this was the development of a reflectorised notice board style equivalent of the fixed distant signal.