I am going to reveal my <i>real</i> ignorance of trading here as I have no idea what you are referring to? Others on this site though no doubt will. In abundance, probably. But entertainment rights sounds intriguing. A quick search reveals The Hollywood Stock Exchange:
which sells the rights to Basil Brush, et al. Perhaps you are thinking of shares in a company such as this? I expect you do know that you can always buy an interest/shares in a West End production and become what is known as an <i>Angel</i>?
I have not meant to be flippant here, so I, with you, will be interested to read replies from more knowledgeable heads. They <i>will</i> write, oldsean, they will write...