There were many different ways that double-slip points were linked to signal boxes but there is one way that has always puzzled me - it can be seen in this 1986 diagram of Penmaenmawr - https://signalbox.org/diagrams.php?id=1144 . This arrangement could never be described as common but examples were quite widespread until branch line and goods yard closures eliminated most of them.
My question relates to departures from the "Down Neck" or "Down Refuge Siding" (I have my suspicions those names are wrong) which actually use the same shunt signal. From the Neck you require to pull 12 & 11 points but from the Refuge you need 12, 11 & 10. So what is to stop you setting for a departure wrongly, and running-through and damaging 10 points?
One possibility would be that 10 points are trailable, I have seen trailable mechanical points elsewhere but they are rarely marked as such on diagrams.
Would the locking allow you to move 10 points if you realised an error had been made?
I am really asking for first-hand information rather than opinion, which is why I am asking about a late example. I don't know when it went, but satellite photographs suggest that area is now plain-lined.
John