Sometimes on a slate roof (flat as opposed to tiled with raised parts) You can bed them flush down on the slate,looks a lot neater to me.All the mortar's underneath the hip tile in a big V shape.
But what your saying doesn't sound right to me.If they did what i said above,you wouldn't see any motar.
As for the overlaping collar's i dont think you fill this in with mortar.I thought these just sloted down on the next one.
I did see these collared ones beded on back to front,so the water would run into the collar,they are still on the roof now.
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