Yes, but the thread wasn't the problem. It replaced the original clutch nut but was over-length and clashed with the diaphragm spring centre.
Found on the old RGM website and in an old paper copy.
[URL unfurl="true"]http://www.rgmmotors.co.uk/main.htm[/URL]
"040373M, CLUTCH LOCATION NUT, FITTED WITH SMALL GARTER SEAL FOR CLUTCH PUSHROD, C, "
"C" = Commando.

Edit:
All Commando mainshafts and clutch centre nuts are the same parts and the Commando clutch doesn't fit the Dominator/Atlas mainshaft.
[URL unfurl="true"]https://andover-norton.co.uk/en/shop-details/15248/mainshaft-gearbox[/URL]
[URL unfurl="true"]https://andover-norton.co.uk/en/shop-details/15024/mainshaft-dominator-[/URL]
[URL unfurl="true"]https://andover-norton.co.uk/en/shop-details/15113/clutch-centre-nut-e6254-a2-372-8652-[/URL]