I referred to them as 2600s as the original question described them in those terms. The GNR's AECs and CIE's were the same design bar a few details, and mechanically (though not at all cosmetically) were similar to the GWR's streamlined ones. In traffic, in railway days, they were collectively always called AECs. Indeed, describing them as 2600s could cause confusion with the modern plastic 2600s, built by Mitsaokorea-SiemensCAF in Cobh.........!
Gardners were early railcar experimenters. With the co-operation of the GNR in Dundalk, they turned out those early articulated units - both the double ended ones and the articulated pairs like the ones Mayner referred to in Australia (of which one at least, I believe, is preserved in operation order; I'd love to see it. I could post pictures of it and they be the right way up.)
The type of technology, the type of design, ised by AEC in Southall and Gardners / Walkers in Wigan were entirely separate and thus "B" was a one-off production and design.
It is to the credit of the DCDR that it has been saved, though it's lucky that CIE's "preservation" of it didn't involve its earlier destruction by the weather, and by certain citizens of our community who took an interest in its aluminium window frames while it was stored in either Limerick Junction or Mallow.
Senior was in Enniskillen when "B"" was running and knew the traffic manager well; his verdict was that the thing was a great success and very reliable. it fulfilled the SLNCR's needs more than adequately. they were only interested in a single vehicle, not any sort of railcar set, as passenger traffic levels almost never warranted more than one passenger vehicle. My late mother's recollections of travelling in it were to the effect that it wasn't exactly full of people. My guess, from what she said, was that typical loads might be 20-40 people. naturally, when there special trains for pilgrimages or GAA specials, more accommodation was needed. This - in latter days at least - tended to involve the company pressing all three of their (by then) seriously neglected bogie vehicles, along with their equally dilapidated 6-wheel brake 3rd no. 4, and borrowed stock from its neighbours. This was often ex-MGWR six wheelers from CIE (bringing green and flying snails into Enniskillen or further), or a couple of elderly bogies from the GNR.
I have it on anecdote that the traffic manager in Enniskillen was mortally embarrassed on such occasions by the state of his own three bogies compared with neighbouring CIE or GNR stock, as the maroon paint had badly faded to a nondescript uneven pink or browny red. The paint was peeling off, to an extent I've never seen on any operational vehicle. The interiors were, I am told, tired, old and musty!
On an aside, I wonder about the green livery of the SLNCR locos in pre-1915 days as shown on Arigna Road. There were several variations of green, and possibly at different times two very different shades used. I'd be interested to know exact details...... certainly, by at least the late 30s (when Senior first went there), all was black on the SLNCR. It looked very stark - but smart - compared with the unrelenting grey of the GSR.