Atlas nose thread is always American 60° form. Original, flat belt driven Boxfords were the same but it was changed to Whitworth 55° form later. I think at the same time as Ver belt drive was introduced.
Have heard suggestions that it went back to 60° form at some time but the three I've handled were all Whitworth.
Parallel register sizes do vary a bit. Nominally the same but "it" might not screw on or it might be a bit loose. Thou or three variation. Depends o the actual machine(s).
Clive