The Right Salute

[inkstain]

If this has been discussed, could someone link me over? I'm trying to describe the heinous rebel-hunter Paulus Catena delivering his last prisoners-of-war to Constantius II in Arelate, and I know that the "Roman salute" as painted by Europeans and copied by the Nazis is wrong, but what is right?
Thanks for any help on this.
Milo