Flange, I'm quite conflicted here. Normally I'd take your contributions as verbatim, but in this instance I'm unsure about the roof tile bit.
Portland cement generally only requires a factory additive when the contract determines it, like the addition of clinker in Ecocem or chem 3 to give additional strength etc. This is mainly seen today on capital projects like speciality bridges and nuclear installations.
BR standards were the only game in town then (and still are imho) but I can't imagine a scenario where Irish cement had a contract to produce such volume of high volume contaminating product that required "boxing off" a rake. Nor can I think of a capital project from 1960 to now that would validate it.
Lagan concrete were, and still are, the roof tile guru's.
My scepticism demands I make a few phone calls and do more research, but it would be fascinating if it were true.
As for the wagon behind the mpd railcar thing, it looks like a donor chassis. Wrong Leaf springs & axle boxes, lack of brake gear, slim buffers and absence of discharge gear suggests it was a trial wagon on loading gauge tests.
This was done with the ammonias too.
Much to think about lads, ye're contributions have been enlightening!