Um, well, I'm in the US, so I better post a picture.
I've also heard it called a barrel bolt latch. West marine has some stainless ones, $60, but the hole pattern is different (larger holes, larger base plate), I haven't found one that will fit in the same space and grab the tab from the seat. My seat has two, one's gone, the one in the photo is pretty corroded into place. It's in front, I'd rather have it in back so the back of the seat with the grab bar is secure. I'll move it if I can but I'd rather buy a second.