The author of the drawing you have been looking at has kindly sent me a copy, and that is a very interesting circumstance indeed.
It is a quite late version of the layout, after the diesel depot came into use, and I had not seen that diagram before. However, I do have a 1929 version which compares rather interestingly. I have posted it at:
https://signalbox.org/diagrams.php?id=1176Although there is a large gap between the dates, the ingoing signal was a fixed flap-disc so this is definitely a historical arrangement rather than the result of rationalisation. There probably wasn't such a thing as a Limit of Shunt Board in the days when that signalling was put in. As I see it, it allows the straight route from the sidings to be used as a shunting neck without allowing such shunts to enter the shed.
But it begs the question of how locomotives actually entered the shed. Was there perhaps authority given somewhere for engines to be called past the fixed signal by shed staff? By the later plan (which is dated 1971) it is possible that all locos entered the shed via the secondary route not there in 1929.
Yet there is also a fixed disc preventing departure by that secondary route. That is not "historical" and its purpose is also worth thinking about. If this is to prevent moves in the depot going beyond it (should we suppose all shed departures were via signal 1 (3/4/6 in 1929)?) then surely a Limit of Shunt board should rightfully have been provided there. Perhaps a look at local instructions would enlighten us.
Whilst I can think of a few places around the country where a fixed disc survived to act as an LoS after rationalisation, the arrangements you have introduced us to are certainly rare. They have a historical background, but it is remarkable such were still applicable in the 1970s.
Best regards,
John