Here are a few postage due items associated with Bristol starting with two postcards from 1922 and 1925 going to the USA with insufficient stamps - resulting in hexagonal "T" tax stamps as they passed through Bristol. The top has a "T / 10 / BS" and the bottom a "T / 10 / 134" handstamp.
The Postcard Rate went up in October 1957 catching out the sender of the postcard below sent in 1959.
Both envelopes below were sent unfranked. In 1964 the top one was charged 5d based on the Printed Matter rate, while the bottom one was charged 6d based on the Letter Rate.
The envelope below, sent in 1964, has an incomplete meter mark so was charged 5d postage due.
Both envelopes below were charged 8d postage due in 1965 and 1966 being double the 4d Letter Rate.