Golden tablets with the seal of the ruler that issued the pass and the seal and name of who it was issued to. About the size of your palm of solid intricately etched gold. The holder of the tablet is backed by the authority of the king to make any required purchases on credit or enter any place they deem necessary. A very powerful token indeed.
A nation was conquered and the conquerers took control and put themselves in all positions of authority. One major problem, the new rulers and the now peasant/ruled class speak entirely different languages.