Mornin' Roger et all,
Further, to add my ha'peth's worth. Apart from the image shown as Plate 50 in Middleton's / Peter Hay's 'Steaming Through West Hants', we've got two reasonable photos of Bournemouth West station, (not junction) Signal box. Both are taken track-side from passing trains, One arriving B'm'th West and one departing, both Copyright.
Apart from these, we've got the BTC 1962 Track / Building ground plan, but, alas, no detailed plans of the S/B itself, or photos from it's south side
The Sketchup drawing of the box was derived from the BTC ground-plan plus the two C/R photos, i.e. by counting bricks, measuring height / length / width / perspective etc.
In this respect, I cannot guarantee it's accuracy, but, it's the best that I can produce given the information to hand.
One (or five) thing(s) that has(have) changed is the replacing of the multi-paned sliding widow frames with panels made up of three large horizontal panes. I'm in the process of altering the drawing to show this.
Regarding the signal gantry, with it's six separate 'Home / Stop' arms, spanning all three tracks immediately west of the box. The most recent images (1926 ?) I have of this can be found at the bottom of page 25 of 'Southern Infrastructure 1922-1934' (Part 1), David Wallis & Noodle Books (Kevin Robertson), 2014, ISBN 978-1-909238-11-2,
and Plate 6 within 'Bournemouth to Evercreech Junction', Middleton Press, 1987,ISBN 0-906520-46-0, This one also pictures the 'Mysterious Obelisk' adjacent to the cabin's entrance, showing that it had been there for quite some time.
See Roger's earlier thread regarding the bracket signal which replaced the gantry. http://www.rmweb.co.uk/community/index.php?/topic/96259-bournemouth-west-signals/
All the best,
Frank.