Thanks all for the information and as I suspected it is probably 8x60 which, to be honest, will be totally fine should it eventuate.

Vintage Canavas, owned a 21H years ago but had been re-barreled to 22/250 by a well known local gunsmith here in Adelaide Australia. Always liked the action but the calibre just was not for me. Eventually sold it without ever firing a shot.

Pick up the 6.5x57 21H tomorrow and hopefully it will present a bit better in the flesh than the pictures. Paid accordingly given it may be a bit of a roughy though. Still, a "well used" M21 is better than no M21!

Cheers