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.

670-1

What you have, with the buckle was a local unit policy and nothing more. It's a morale booster and nothing more than that. Once you pcs away from that duty station/unit you can no longer wear such a device UNLESS your new unit allows such a device, which is doubtful.

Amazing

Not to beat a dead horse, but unit COs can authorize slight uniform changes. And to some degree, even a 1SG can dictate policy as they are the ones that enforce standards. I had a pair of boots that were issued to be mountain warfare evaluated. They were authorized for wear by the CO after the trial period. Company got a new 1SG and they needed approval again. Transferred to another unit in a different state, authorized again by that CO and 1SG. Company got a new CO, and I wasn't allowed to wear my mountain boots anymore with the new chain of command.

AR 670-1. Everything else a commmader might authorize as local policy but once you are outside of his/her sphere of influence you ought to go back to the Good O’le Army Standard!

Another example...I was in the USMC, and switched over to the Army. I liked my pointy cover/headgear. I could not wear my USMC cover in my Army uniform. Same with switching units. Semper Flexible!

Can you find anything saying it is authorized? Can you find anything saying the belt is authorized in your new command? Can you find a regulation that says you can wear a purple striped hat? If you can not find it authorized, it might not be authorized.

Nothing specific in AR 670-1 nor DA Pam 670-1. My only thinking is that in your old unit, there was a Troop/Squadron/Regiment policy allowing such and now perhaps in your new unit, there is no such policy.