Found it! Here
is a scanned picture of the platform ground frame at Bethesda - the accompanying "blurb" says that it was taken in 1927.
It seems to be looking out over the goods yard - opposite what would be the cattle dock. The "box" diagram is titled "Bethesda Station". That blurb also calls that box on the fence a "telegraph instrument" - I see maybe a bell tapper but it doesn't have any block indicator that I can see. Also in the text is mention of this frame controlling points signals and "crossing gates". But that seems improbable - I can't see any crossing within range of the platform based on the OS map. There is a shadowy blob on the platform at the LHS of the frame that is shaped like the head of an Annett's key vertically inserted into a "lock" - is the picture showing the frame unlocked?
There aren't any nameplates on the levers, but they seem to be differently colored - all "shades of grey" to coin a phrase... I see light, light, dark, dark, light, dark, space, space, dark, dark - those last two are reversed.
On the diagram you can just make out two pairs of ?crossover points? both facing SW-NE on the right lower end (one partly under a lever). <edit - maybe it is one facing point partly under the lever #4 and a crossover - that would make it more likely to be the passenger loop, but would also fit the goods RR>. Then there is also at least a crossover to the left upper and several suggestions of points in that area as well.
If you compare to the 1889 Map
the diagram doesn't seem to all be quite right in several respects to fit all the aspects of that layout. The situation in the Goods Yard seems a little more complex than I would have expected. There's an ability to run-round completely within the yard, I think, and a through line in the goods shed that ends in a head shunt accessed via crossover over another line? Nice to see a prototype for the commonly modeled reverse road into Gas works siding!! And a handy crane near where the long sidings and the good shed line converge. The turntable has a coaling line - just space for a coal wagon but no coal dock, perhaps, and also a space for an engine to stand beyond the turntable - but no shed.
Not sure what we are to make of all this. The signal box on the same 1889 map that other people mentioned seeing would have to be beyond the end of this map. At the LHS in the photo's diagram the lines do not seem to be converging like the map shows them to be - supporting the notion that a signal box controlled the divergence point and this frame controlled shunting?