Thank you very much!
We've got a whole cupboard of those arbours at all different sizes – 10 or 20 in total – so need a set of tools for them or one adjustable tool.
Thanks for mentioning mill arbour spanners – agree about the 4 points – all the mill arbour spanners that I could find had 4 points not 2 and the geometry of the points was different.
An adjustable pin spanner might well be a way to do it – I hadn't seen the characteristic two round holes for a pin spanner so I hadn't thought of that and the adjustable pin spanner I have here is for much smaller, lighter things.
There also seems to be a much heaver duty version of a pin spanner – an adjustable gland nut wrench – intended for the glands at the ends of hydraulic cylinders – so I've ordered one of those. The pins are round not square so not a precise fit but might be good enough. I'll see how things go with it and if it doesn't seem robust enough I'll turn down a piece of stock to the right diameter for each arbour then mill the two teeth in the ends of those.
Thank you all very much for letting me know that they're proprietary – I was beginning to wonder if I was going crazy not knowing any standard tool for them.
The original tool or tools got parted from the mill and arbours before my friend bought them.
The mill is a Reiden HF30 Mill: