Answer - they're all unmarked. Uniquely, US web addresses contain no explicit indicator of nationality, and ditto for UK stamps. In both cases there's an easy historical explanation: these are the places where these inventions originated, so they didn't need to distinguish themselves from anyone else. As for LF, it appears to be the only genre of fiction that isn't referred to as a genre. Indeed, people who read and write other genres frequently refer to the other kinds as "genre fiction" specifically to distinguish them from LF. I heard a lot of this at Readercon, and to be honest I was a little taken aback.
The explanation in this case is much harder to come by. What makes LF different from SF, Crime, Romance, etc., in this regard? Here are a few possibilities.
a) As with UK stamps, LF came first. Except that it clearly didn't: the Epic of Gilgamesh would never have made the Booker shortlist. Even bringing things up to more modern times and looking at the roots of what is now called literary fiction around the turn of the 19th century - we can equally see the roots of SF, romance and horror.
b) It's purely a publishing and marketing designation. Certain houses take on certain kinds of fiction, and book buyers like to know which section of the shop to go to. Genre is a useful label for professionals and punters alike. Well sure, but that applies to LF no less than to any other genre. Except that when it's LF, the booksellers don't call it that - they just shelve it under 'Fiction'.
c) LF isn't a genre in the way that the rest are. This is an argument I've heard made, though it's hard to stop it from lurching into an insult that defines LF by characterizing other genres as mechanically formulaic. We hear this kind of assertion at its crudest near the borders, where people are trying to assert their LF credentials - in Atwood and her talking squid, or Pullman claiming that most fantasy is a "shoot 'em up game". Insults to other genres apart, to me it seems ridiculous to assert that LF isn't a genre, for if that were the case how could anyone know when they were reading it? But know they do - even if they are not conscious of the criteria they are deploying to arrive at that perception. The only way I can wrestle sense out of the argument (at the risk of bringing back the insults) is by imagining that what distinguishes LF from the rest is not similarity of form or content but simply quality - that it's a kind of fiction Super League of books that have nothing in common except being very well written. But that notion falls at the first fence, for much LF isn't well written at all - especially if we take good writing to include such fundamentals of fiction as pace, plot, character, etc., and not just the crafting of individual sentences. I have been bored to tears by many dull and formulaic books over the years, and LF has featured prominently in their number. Alternatively, one might suggest that LF is defined by being more experimental, treading new fictional territory - but again, the vast majority of it just isn't - and it would be difficult to sustain an argument that, say, SF has been less experimental.
Well, I'm no doubt preaching to the converted here, but that's precisely why I was taken aback to hear people at Readercon refer to themselves as genre writers - because to do so seemed tantamount to accepting LF as a uniquely - indeed oxymoronically - genreless genre. Not that there was any obvious sense of self-deprecation in their use of the word, but still. (For similar reasons, I've never felt entirely comfortable with the American phrase "person of colour", which to my ears suggests that (so-called) white people have no colour - i.e. are the default. But that's not my call!)