Mansfield North Junction - 1920
The 1920 drg is from a MR apportionment diagram.
It seems that there is some interesting history for the bracket shunting signals 17/18/20. A drawing of the 1950 layout, by the Signalling Record Society, shows those signals as a 3 way indicator and this fact is borne out by a 1948 photo here
www.flickr.com/photos/192151030@N08/52031296555/in/album-...
However, in the notes here www.flickr.com/photos/192151030@N08/52031918745/in/album-... we see that on 25 January 1953 the 17/18/20 bracket was renewed as triple discs.
So it appears that, sometime between 1920 and 1948, the 3 arm bracketed shunting signals 17/18/20 were replaced by a single arm with 3 way route indicator and it is this that was replaced by a 3 vertical disc set on 25 January 1953.
Mansfield North Junction - 1920
The 1920 drg is from a MR apportionment diagram.
It seems that there is some interesting history for the bracket shunting signals 17/18/20. A drawing of the 1950 layout, by the Signalling Record Society, shows those signals as a 3 way indicator and this fact is borne out by a 1948 photo here
www.flickr.com/photos/192151030@N08/52031296555/in/album-...
However, in the notes here www.flickr.com/photos/192151030@N08/52031918745/in/album-... we see that on 25 January 1953 the 17/18/20 bracket was renewed as triple discs.
So it appears that, sometime between 1920 and 1948, the 3 arm bracketed shunting signals 17/18/20 were replaced by a single arm with 3 way route indicator and it is this that was replaced by a 3 vertical disc set on 25 January 1953.