Dear Alexis,
sorry this is not much; i hope to find out more and add it later. Outwardly this is the breastplate of Dharmapala's suit of armour as worn by his oracles, and if i remember right, yes it is adorned with the seed syllable HUM as we visualize in our yidam's heart, or in the hearts of Dorje Shugden's four activity aspects during mantra recitation.
It appearing like a mirror could refer to one of the main 'jobs' of Dharmapalas - transforming anger into mirror-like wisdom; i'm not sure about this one but at least it makes sense!
yours, beggar