Daryl;
To be totally honest here I really don't know. As I said mathematically 18mm calculates to a 13ga. I do not know if they produced a gun with a misnomer of 18mm similar to the 14mm & 12mm for the 32 & 410 or not. It could of course simply be a 12ga with an undersize bore. Nominal bore for the 12 in metric is 18.5mm.
I have an old Birmingham proofed W Richards (Neither Wesley or William) hammer gun with 12ga chambers & the bores are marked 14. In metrics it would thus be somewhere between 17.6mm & 18mm.
