Where can I find any standards regarding a belt buckle authorized at a previous duty assignment?

Things like belt buckles and hats are authorized by the unit at the unit level. It's not that you can't wear it because you left the unit. You're not authorized to wear it at all. The Commander of the previous unit authorized it to be worn for members of the unit.
The standard is that 670-1 defines what belt you are authorized, and that commanders are authorized to make changes to uniforms at their level. Once you've left that command, you know longer fall under that uniform authorization.

The buckle nor any other adjustment to the uniforms outside of the AR are not authorized. Local commands often use Belt buckles as a moral booster because they are not generally visible. Your local command shields from UCMJ, once you are out from under hat umbrella, it falls right back to the AR. In fact if you are in a unit with such buckles and you attend an academy or other significant training away from your home station you would not be authorized in that instance either. Take it off and place it I. Your future I love me kit. Thank you for your service.

Know it's not what you're looking for, but in the Navy, command belt buckles are authorized uniform items beyond the unit presently serving in. However, I don't think (anyone?) that command ball caps can "move" to a different unit.
Just another service culture difference.