If an adjective, rather than a proper noun, we could perhaps allow a lower case "b," no? As in "Königlich bayerische Post." Here's an example of that phrase that I found:
Die königlich bayerische Post
https://www.markt-velden.de/die-koe...-velden.htmlClick on that URL to see the same phrase there as well. It was the official name of the Bavarian postal service.