My understanding, but in no way am I am an authority, is that facing LEFT was a sign of war. Facing Right was peace. However, this is my rambling readings from all over history...
Problem with this theory Al is that there was relatively few eras of peace, so that there should always be left facing portraits.
And, my apologies to @vespasian70 , your OP is fantastic! I really enjoy the history behind Vespasian and your issue is gorgeous! Nice capture!
I very much agree with that. However, I believe that the role of the Principate was to show that the Empire was at peace, and all was well. Kinda like a propaganda...all is well on the Home Front...
I can't remember where I read it, but it was suggested that perhaps it had to do with the celator being right- or left-handed. Interesting speculation anyway.
During the Flavian era types were struck with both right and left facing portraits within the same issue. The "war and peace" theory could not apply here.