I believe I have a possible/probable (?) unit for Henry. Jim1 will be familiar with the black art of checking records for numbers near to the subject's for similarities. This is made easier in his case because of the earlier information that told us that about a re-org of numbers within the railways element of the RE.
Firstly, RE numbers around the 1749** area are as far as I can tell, almost all railwaymen of one sort or another and the RE railways medal roll in which 174955 appears has a whole slew of men, in alphabetical order whose numbers are neatly in numerical order below and above that number. That accords with the information that each unit provided its list of men who were to receive new WR numbers.
Secondly, and perhaps more persuasive, I find:
174945 2/Cpl Acton becoming WR 256557 unit 275 Railway Company
174960 Spr Lay becoming WR 256571 unit 275 Railway Company
Two others 174946 and 174970 who did not get a WR numbers were also in 275 Railway Company.
While the evidence is circumstantial, in the absence of anything else, and with Acton and Lay neatly bracketing (artillery term!) Henry in what is clearly the nominal roll of a unit, I would invest £3.50 in the war diary which will at least show what such a company got up to and, if you are persuaded, could be seen as describing some or part of Henry's war experiences.
http://discovery.nationalarchives.gov.uk/details/r/762f221ccaf146e5ae36c30366ed31a2maxD