The great thing about the Blue Square multi working, plain twin-pipe vac brakes and buffer and hook couplings was it enabled great flexibility in forming trains. Something I'm sure Ayr depot was grateful of after the fire gutted several units.
I know it's annoying when someone comes along with points like this, but Ayr was about the one depot where this didn't apply, as the Swindon-built cross-country units which were the bulk of its allocation were the White Circle coupling code, built with some air pipe differences for a reason which seems never to have been given.
Having said that, I once took a Glasgow-Ayr service which was a normal 6-car Swindon unit, plus a 3-car Glasgow area Derby Blue Square unit at the back. I always wondered how that was done, and discussed it here a few years ago only to be told it was not possible; that I had mis-identified it; that I had not noticed the rear unit was not coupled and was left on the buffers at Glasgow, etc. All wrong.
The Blue Square system was, I understand, devised for BR under subcontract by Walker Bros. of Wigan, who also did various other diesel unit components, and even a few narrow gauge complete cars. The previous Yellow Diamond system, also theirs, was much the same, apart principally from the engine starting sequence, and could have been readily converted without too much rewiring, but that was never done.
Incidentally, regarding the Gloucester bubble cars, the predecessor 1940s GWR single units (cars number 17-38) they replaced were also built there under subcontract from Swindon, as were the Gloucester cross-country units as well. Gloucester RCW were a longstanding subcontractor to Swindon, I believe the construction drawings were issued by Swindon, and even the correct BR shade of green paint was sent up by the Swindon stores.