The article by Richard West discusses and pictures some of the trials that were printed there. As you can see in this illustration, there were several texts that were tested for the 100th anniversary of postage due labels in 2014. Also, apparently there was some discussion of noting the centenary of India airmail that same year, though it ultimately wasn't done. That trial is in the middle right in the illustration.
By the way, if you are not a member of the Friends of the BPMA (soon to be The Friends of The Postal Museum), you should consider it. Now that the new postal museum is becoming a reality, membership will keep you informed of progress as well as supporting the museum.
Cross Post is an excellent journal with a variety of articles. In the August issue there is also an article titled "A Curious Culture of Letter Writing," which describes how postal reform and the postage stamp revolutionized Victorian Britain. Much of it is familiar to those of us familiar with British postal history, but it's a good summary with some interesting details and illustrations.
For example, did you know that in November 1840, Rowland Hill suggested letter boxes for collecting the increasing volume of mail? His suggestion wasn't taken, and it was more than a decade later when Anthony Trollope suggested it again, this time successfully.
Information on how to become a member of the Friends is on the BPMA web site.