The wikia and gamepedia pages seem to say that the Desert Rangers had the Riot Gear (duster included) first, and the NCR followed suit after the Unification Treaty. Which is confusing because it suggests that not only was it LAPD riot gear, but that it was also used in the war with China (the Survivalists version has Nanjing and Shanghai written on it). There's also the giant statue commemorating the treaty, which has the duster wearer standing closer and with their back to NCR territory. Note that the patrol armor isn't the combat armor we saw in Fo2, so it's also new. So wouldn't it make more sense if the NCR were the ones that had it first? I think this would make the chronology of the armor less confusing. They had LAPD armor in California, the NCR Rangers started using the rare armor for veteran rangers, the NCR and Desert Rangers developed a treaty, the patrol armor was added to the repertoire (while any new Veteran Rangers would then get the riot gear as well), and some riot gear was found in Hopeville and Ashton. Instead of the NCR Rangers used combat armor, and then patrol armor, and then made a treaty with a group based in Nevada--which resulted in them wearing LAPD riot gear from some town in Death Valley before they even went there.