Thanks for your thoughts on this.

The seatstays are plain. No marks on them.
Re BB shell, I hadn't thought about that. You're quite right. I thought that no cut-out means fake, but yeah, the plastic thingy was probably there.

I'll take a couple more photos these days and post them.

I forgot to mention, the rear brake cable is routed through the top tube through little diamond braze-ons.