If your thread gauges don’t fit, that is a good indication it is BSPT since the threads are 55 degree Whitworth profile and won’t match up very well with 60 degree thread gauges. Measure the OD at both ends of the threads and see which measurements are closest from the link I posted earlier and you should be able to determine the fitting size. My guess is it is either 1/4” BSPT, or 3/8” BSPT based on the pictures of your mill and the pump on the PM site.