[quote="L_Marshall, post:8, topic:340528"]
Maybe Fr. Z?
I found this blog post in which Fr. Z writes:
Bishops don’t, or shouldn’t, say Low Mass in the manner of a priest. They have their own ceremonial to observe. For example, they are accompanied by their secretaries and they vest at the altar.
The underlying idea behind the different and more solemn Pontifical Low Mass seems to be theological. They act for the diocese and the larger Church when they say Mass in a difference way than a priest would.
Still, this is only a blog and he doesn't cite any sources. I would look for something more authoritative for a definitive answer. Glancing through the 1960 rubrics I didn't see anything specific, but it might be there--or somewhere else.