Here is something connected to the topic, just a little different.
In 1994, Royal Mail agreed to supply Boots the Chemist [chain store] with a first class stamp to be included with the sale of greetings cards. The First Class Machin definitive was issued in a small pane with diagonal roulettes, which were folded for insertion into the greetings card pack [visible on the corner for ease of sale].
Unfolded panes were available from the Philatelic Bureau.
The Boots Logo appeared in the Margins and the pane was printed in Litho by Questa, Roulette 8.
Later, other outlets joined in this idea and panes were printed without the logo.
In 1997, the printer changed to Enschede of Holland and the Roulette was 10.
They make an interesting addition to ones GB collection....or Machin collection.
But what has this to do with Exhibitons ??
Some while later, enterprising [or not] dealers jumped upon the idea to use such panes to celebrate events. Permission was granted to overprint in the margins and some very interesting Exhibition material was born, available from dealer stands.
They became quite popular especially because of the types of pane. Overprints were found on panes with logo, without, 8 and 10 roulettes etc. Another collectors nightmare....but an interesting one.
Here are some from my collection...sadly not yet complete.
Pacific '97

Moscow '97

Singapore '95

And Singapore'95 printed on the gummed side !

I will show more sometime. They are quite attractive. Many scarce items exist with different colour overprints, missing phosphor panes etc.
It's all good fun isn't it.
Londonbus1.....I will try a gold overprint next !