I think the CEJ is intended to be used like a serial tap to reduce the tapping loads on these big BSW threads.

Start with a taper tap and run in until it gets to be really hard work. Probably no more than a turn or so at full depth thread. Run the undersize CEJ in and finish off with a full size bottoming tap. If a set were made it would replace the second tap. But I don't think serial sets were a thing in those days.

Being undersize the tapping loads are lower and the tendency to pull sideways is reduced.

Big coarse BSW taps are a pain to keep true when used by hand. Doesn't help that the taper on the taper tap is often excessive so its difficult to get them cutting well enough to pull in. I have one biggie that is basically a reamer. Have to step drill the end a thread or two deep to get it to start by hand. Leaning on a tapping head in the big Pollard drill teaches it manners tho'.

Clive