I have a little corollary to this discussion - perhaps it should be in 'Overseas', but it is germane to the No 7 Auxiliaries. If we go to NZ, I suppose the 'home' of No 7 Tablet instruments there were installations called 'Tablet Transmitters', not as you may think intermediate instruments, but seemingly an auxiliary pair to an intermediate siding.
Consider a section: Lovells Flat - [Stirling] - Balclutha. Lovells Flat and Balclutha are terminal instruments, there is a pair of 'tablet transmitters' between Stirling and Balclutha. The 1974 operating instructions read:
a) A tablet is obtained from either Lovells Flat or Balclutha for the Lovells Flat - Balclutha section, using bell signal Release Tablet for Shunting (5-2), followed by the usual Train Departure (2) signal.
b) On arrival at Stirling the driver gives the guard the tablet, which unlocks the points. The guard then puts the tablet in the top slide of the tablet transmitter instrument S, closes up the instrument and sends Train Arrival (3) to Balclutha (where the other tablet transmitter is located).
c) Balclutha closes up his tablet transmitter B2, and then requests the guard at Stirling permission to release a tablet from B2 by sending the special bell signal Transmit Tablet (4-2). He then inserts the tablet from B2 into B1 and sends Shunting completed, Tablet replaced (2-5) to Lovells Flat. After Lovells Flat has closed his instrument, ordinary tablet working can be resumed through the Lovells Flat - Balclutha section.
d) To bring the train at Stirling into the section again, the guard telephones Balclutha who then obtains a tablet from B1 after sending the signal Release Tablet for Shunting (5-2) to Lovells Flat and obtaining a release. The tablet is then placed in the top slide of B2 and Train arrival (3) sent to the Guard at Stirling.
e) The guard closes up his transmitter S and sends Release tablet for shunting (5-2) to Balclutha; on obtaining the tablet from S, he unlocks the points and reverses them, moves the train on to the main line, restores to normal and relocks the points, and hands the tablet to the driver (Note that the tablet instrument is left in the unclosed position)
f) When the train arrives at Balclutha or Lovells Flat instrument there, which is then closed up, and Shunting completed, Tablet replaced (2-5) sent.
Personally, I think these are 'straight' instruments, set up to work as an auxiliary pair; the three-position visual indicator was removed, and a plate reading 'Tablet Transmitter' put in its place.
Also of interest, and something that isn't often recorded in UK installations is the number of tablets per instrument: each Tablet Transmitter had four tablets for each machine with another nine for the operation of the main section up to a total of 36 tablets for the section, normally there were 24 tablets although there were cases where there were 30 tablets per section.
Seems that Tablet Transmitters were a bit more popular in NZ than auxiliaries in the UK - 11 known installations, two of them temporary (Paeroa area during recovery of the old line, Mangaroa during the construction of the Rimutaka Tunnel) and one Smart Road to Lepperton which had two tablet transmitters in the section [if this meant that one end had three instruments in the same box or that there were two instruments at the one end, I don't at this stage know.
