I California we usually see U/T/A - Under Trust Agreement or U/D/T Under Declaration of Trust - but as Mr. Frederick said, U/A is fine too.
We usually see U/T/A as well.
To further clarify, since we are bantering TYPE A individuals, U/T/A, U/T/D, U/A or whatever else you use are not legal abbreviation per say. They are essentially created for convenience and universally recognized by estate planning attorneys so you dont have to write out the full words.
Very true. Not sure that either one of us are Type A, though. I prefer "detail oriented!" ;-)