Pete.
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- Kent, UK
Well you do have the option of simply re-making those posts to the measured diameters and pitches. Or if you want to simply clean out the internal threads you might single-point some silver steel and harden it to make a triangle tap which would clean up those threads nicely.It's honestly finer than it looks, I've got Whitworth thread gauges and 30 or 32 is the best fit. Metric 0.8 is a better fit! It's maddening. I have some BSF taps and they don't fit well.
Back in the day many people make their own cutting tools and it might simply be that someone made a tap, then used the tap to make a split die then produced all of those posts with the tools they had made for the job.
Meanwhile, attached is a handy look-up reference for a lot of historical (and current) threads.