[SteveCG]

Indeed John, it is all a bit of a puzzle. Studying some of Triax's pictures closely it may be that the rigger just did not bother to use the trombone mount supplied with an 18element aerial and so the use of just an end-mount for this size of aerial was never Triax's intention and hence a red herring for me...

I think there was a label on the boom - there is a patch at the shortest director end of the boom which looks suspiciously the right size - but not longer legible.