Mike Hodgson wrote:Vol. 2 of An Historical Survey of Selected LMS Stations by Hendry & Hendry has (undated) diagrams for both Bromsgrove and Blackwell boxes and comments on several of the signals and operating practices, although there is no mention of permissive working. Data for the diagram of Blackwell is credited to Mike Christensen. The subsidiary signal the DI mentions is shown as 39, but with no mention of the C/W. On the Up Main, signals at each end of of the platform are shown as having W subsidiaries.
It is possible that the calling-on facility (along with the methods we are discussing) was later withdrawn - I have two plans and the later one shows only a W. No guarantee that is accurate, of course.
However, a 1956 drawing definitely shows the C/W - see http://www.signalbox.org/diagrams.php
- although it is very small and you may need a microscope!
I wouldn't really call it "permissive working" as the railway clearly regarded it as "Locomotive Assisting in Rear". Perhaps I should clarify that "Locomotive Assisting in Rear" circumstances listed in the SA are not the same as "Locomotive Assisting in Rear" in the signalman's regulations, which would be better titled "Locomotive at Rear of Train". Engines would have simply been signalled into the section behind a train with the appropriate number of 2-2 bell signals. The thought of doing that for five Jinties . . . they'd probably be arriving at Bromsgrove before you'd finished your campanology! (Big Bertha must have made life a lot easier) And with the option of sending more in behind existing ones, the man at Bromsgrove would need to count carefully before he got to the stage of giving 2-1.
The warning signals on the up line were specifically provided to allow trains to draw down to the starter without applying Rule 39 which would certainly make the signalman unpopular!