Those buckles can’t be that rare; I recently saw one on a young girl’s belt decorated with hot-pink skulls and crossbones. That buckle was painted black and probably not even brass, though.
Also mine, 20 years ago.
The buckles you are issued in boot camp are brass and indeed require a lot of care to shine and maintain. Basically you shine it, put it on, then shine it again to get the fingerprints and oil off. The shine is good for about…the rest of the day if you’re lucky.
One you get out of boot camp, you replace ALL, and I mean ALL of your brass with the anodized version which mostly stays shiny with a few regular wipes of an old skivvy shirt.
You won’t get your hands on the anodized stuff in boot camp, and if you did it’s be spotted pretty quick.