Thinking about it, could it just be that 11 lever was always intended to be spare? From what I recall, some of the IMR frames I've seen had spare levers for no apparent reason past or present (I'd have to look at photos I've taken to see what ones).
An example of a similar situation to the proposed scissors crossover and actual layout at Warren Street were the Scissors crossover at Clapham Common and (later) single crossover at Stockwell Northern line.
Clapham Common (T)
A scissors crossover north of the platform
I don't have the lever details for Clapham Common, but the signal diagram shows the following numbers which I assume would relate to levers:
1 SB Outer home
2 SB intermediate home (junction signal) route 1 (to SB platform)
3 SB inner home
4 SB starter
5 Shunt signal NB main to SB platform (for N-S reversing)
6 SB intermediate home (junction signal) route 2 (to NB platform)
7 ?
8 ?
9 Crossover from T6 to NB platform (route 2)
10 Crossover from SB platform to NB
11 Wrong direction starter SB platform (SB to NB)
12 NB starter
13 NB Inner home
14A. 14B NB home signals
15 first NB controlled signal
This would have made it a minimum 15 lever frame. I assume that one lever would be the release lever and the other may have been spare.
Stockwell (U)
A single crossover north of the station
Lever numbers below
1A Outer home SB, 1B Inner home SB
2 Spare
3 SB Starter
4 Spare
5 Colour signal NB main to SB platform (for N-S reversing)
6 Release
7 Crossover
8 Spare
9 Wrong direction starter SB platform (SB to NB)
10 NB starter
11 NB Inner home
12 Spare
My understanding (which may be wrong) was that the Stockwell frame came from Clapham Common. If so, and Clapham Common had 15 (or more) levers, then it must have been cut down to 12 before installation at Stockwell.
Warren St (VG)
According to the signal diagram for the single crossover, Warrent St would have had the following levers (Hellocontrol can fill in the blanks). Green entries are potential lever use:
1 NB home
2 NB starter
3
4 NB wrong road starter
5
6 Crossover from SB platform to NB
7
(Crossover from VG11 to NB platform (route 2) )8 Colour signal NB main to SB platform (for N-S reversing)
9 SB starter
10 Inner home (no aspect)?
11
(Intermediate home - junction signal)12 Outer home
Looking at the diagram and the numbering sequence, logic would say that 11 could possibly have been the provision for an intermediate home signal (the junction signal) VG11. This would tie in with the arrangement at Clapham Common where there was an outer home and then the intermediate junction signal at the crossover.
It appears that VG12 is sited at a distance sufficient to protect a train reversing S-N over 6 crossover.
In the following scenario:
A train (T1) is going to reverse S-N via the SB platform and 7 crossover
There is already a train (T2) in the NB platform that will be carrying on NB.
An additional signal (VG11) at the start of 7 crossover would allow T1 to get closer to the crossover so that as soon as T2 has cleared the locking, 7 crossover can be reversed and VG11 cleared so that T1 can move. This would save the time that it would take for the train to move the distance between VG12 amd the crossover if the train was held there. Whilst the same saved wouldn't be much, when moves are measured in seconds, that could make a lot of difference over the course of several reversers.
Or there again, perhaps 11 lever was just an additional spare