Mixo horns at least are mirror-images
With the trumpet facing forwards and the spades uppermost ...
... the curve of the trumpet is downwards (as indicated by the arrow), so anything that gets inside will lodge there.
This is what came out of just one of mine (not the tape measure, that's just to indicate the scale ...)
They need to be mounted such that the curve of the trumpet is upwards (see arrows), which will resist water and debris getting stuck inside, even though this puts the spade connections at the bottom. This can be partly alleviated by angling the open end of the trumpet downwards to some extent, which will further help avoid muck getting inside.
This drawing from the 77 model year and later Leyland Parts Catalogue shows the spades on the opposite side to the trumpet, which means the trumpet has to be facing forwards (or backwards) or the spades foul the bracket. Clausager shows a 69 model (p63) with them pointing forwards like this.
However both the Mixo horns on the roadster and the different make on the V8 here have the spades to one side which means they can be mounted so the trumpets face across the car and avoid the worst of the weather. Clausager shows a 72 model (p65) like this, but bear in mind the horns in his pictures may not be original, as mine aren't.
For early cars with a sub-harness to connect the second horn you need the horn on the right-hand side of the car at least to have double-spades on each terminal, like these original BHA4514 and BHA4515 horns from Leacy.
But if you have the later single-spade per terminal horns you can avoid mangling the harnesses by using a male-male-female spade adapter. Loads in America apparently, but I've not been able to find any in the UK, or even China (which is surely where they come from). I'm not paying the ludicrous postal charges that seem to be the norm from America, so looked at making some up. It occurred to me that I could use a piggy-back (one male and one female) and a male spade, back to back. Conveniently my piggy-backs have an second tube around the main crimp tube, and extending back from it, so with the insulation stripped off both the back of the male slots neatly into that. A copper nail that extends through both components, crimped, soldered and the whole thing heat-shrunk makes a neat job.
Subsequently I came across these with Google. 6.3mm from several sources - UK, two in America, in side-by-side or back-to-back configuration, another here in side-by-side configuration, and one in Australia.