Very interesting. I looked again at the Derry photo and remembering Mayner's comments that it may be a different chassis altogether for the first batch of wagons which this was
http://catalogue.nli.ie/Record/vtls000307375
To me it looks like there is a vacuum hose on the left of the three-link, more similar in appearance to some of the flat wagons than the bubbles, with a pipe travelling obliquely above it down the near side of the chassis, turning obliquely in again at the far end
PS Sorry about three posts, something is acting up on the iPad, couldn't merge them