I remember Compo opening the box and shaking salt on it. I think it was used to make us think about it and torment us because we'll never know. 8)
I thought to do a wee bit research to determine into which theatrical effect by technical term the dreaded matchbox fitted. It would appear to be a 'Trope' and specifically an 'Ostentatious Secret'. I found a fascinating website covering the many 'Tropes' in our beloved show, TV Tropes at:
http://tvtropes.org/pmwiki/pmwiki.php/Series/LastOfTheSummerWine?from=Main.LastOfTheSummerWine
I will try to add here what it says about the Ostentatious Secret of the dreaded matchbox but the dreaded mind police might inhibit this as it does mention a part of the male anatomy - that what Compo got all shy about when the phrase 'your person' was used. So, if it does not show, go to the website:
Ostentatious Secret - Compo's matchbox, with its unknown but terrifying contents. Joke shops in England used to stock matchboxes exactly like that one, each containing a rubber severed penis.
Go to the website anyway to see the many interesting technical terms for features of the show. Three I like among many and by way of example:
Ascended Extra - probably one of the biggest culprits of this. The characters of Wesley, Smiler, Auntie Wainright, Billy and all three policemen were originally one off appearances, who later came back as regulars. Countless other minor characters were increased in importance (Alvin, Billy & Entwistle graduating to members of the main trio).
Pet the Dog- Fierce as the ladies were, they were all given their tender-hearted moments. This mostly happened for Pearl, arguably the meanest female on the programme, who was noticeably keeping an eye out for the widowed Clegg, particularly after Compo died.
Put on a Bus- Mostly when one of the cast would die, the show quietly killed off their character, but mentioned the death in passing. However, when Kathy Staff died, her character, Nora Batty, packed up and moved off to Australia, and was never mentioned again, despite having been one of the show's iconic characters.