Dan,
BS 46 an very early 1950's standard, so it must be assumed before that it was down to the "Works Engineer" to devise the standard such items were built to. You can imagine the rivalry between the different manufacturers when claiming that "their system" was better than others.
So, you are right of course to be wary when working on very old equipment and trying to determine what standards they may have be built to. I would suggest that there was no standard given that it was not until the 1840's that Sir Joseph Whitworth developed the first standard for screw threads and went on to develop the first screw cutting lathe in order to make standard nuts & bolts. Up until that time screwed components were made by individuals and no two would match another persons. Standards for other components would also not of have existed.
I recall as an apprentice I had the opportunity to do a little work on the a full size replica of a steam locomotive. It was a replica of "Locomotion No 1" built in 1825 to mark it's 150 year anniversary. Locomotion No 1 was built to haul the Worlds first Steam Public Railway : The Stockton- Darlington in 1825. On the original, the bearings were fastened to the con-rods by a system of yolks, keeper plates & wedges, no two assemblies were the same & certainly not interchangeable. When replicating the parts we used tools and equipment that simply did not exist 150 years ago & ours just looked to "engineered" to convey the period. So, for the first time as apprentices, we were asked to make rough looking parts!