N.B. This post was originally written about a mystery, but the mystery is now solved. See the wonderfully worldly description in the comments section.
Now, I'm proud to say that I have a depraved and horrible mind. I know what felch means, I know what changing at Baker Street is, and I know at least 5,636,336 synonyms for the word perineum. But I have no idea what the Emperor's Crown might be. So I googled it. And what did I find? Nothing nil zero zilch rien.
It's actually a rather important point in the novel, now lost to understanding.
There are several competing problems with understanding the language of filth. The first is that obscenities are not written down enough, except on the walls of lavatories, which are then demolished or repainted. The second is that those places where they are written down tend to be terribly dubious. There is, for example, a splendid book called Roger's Profanisaurus that lists rude words and their meanings. It is revised each year in time for Christmas and then appears on the tables in Waterstones. Yet ten years of revisions have not got rid of the fact that a chap I know spuriously entered the name of another chap I know with an obscene definition. It's still there. I check every year.
The Urban Dictionary, which is in some ways terribly useful, is the same. There's an Auden poem that opens:
Doom is dark and deeper than any sea-dingle
I happened to mention this aspect of doom to Mrs Malaprop and she didn't know what a sea dingle was. I reached for a dictionary, she for a computer. I found that a dingle was another word for a dell (hence Dingly Dell in Pickwick Papers is a tautology). She found this definition from the Urban Dictionary:
A sex act involving two people in which salmon roe is used as lubrication
Now, that ain't true. I know that ain't true. That's somebody who's read a little too much Auden (if that's possible) and is having a laugh. The problem is that the two best resources for the investigation of obscene English are tainted, infected. They have informational herpes.
And where does that leave the Emperor's Crown? What were the three nice girls doing so admirably? Perhaps Graham Greene was making it up. Almost any words in the right context can sound obscene. Have you ever done a flick Geoffrey? A koala gherkin? A Dutch steamboat?
If you have any idea what the Emperor's Crown is, do leave it in the comments. Otherwise the details may be lost to future annotators of this great novel. All that the prudish, priggish web would tell me was that the Emperor's Crown is usually bestowed by the Pope.