Hi Freek,
This won't help, but I didn't want you to think you'd been entirely ignored.
I was just comparing your Rauler to my NOS 1983 Rossin Aerodynamics. Strangely similar. My tubing is also 'teardrop' as you describe. I would have said, in section. similar to the pattern the chain makes on a single speed bike. On my bike the top tube has the smaller radius on the top of the top tube, on the bottom or front of the down tube, the front of the seat tube, on the bottom of the chain stays, and on the front of the seat stays. Mine has the internal rear brake cable, but just holes in the top tube, no braze ons. My shifter cables are external, I've never seen any like yours. The rear dropouts are similar, and my rear derailleur housing boss is where your cable exits the chain stay. My lugs are similar, but slightly more ornate. And along with my theme of being no help, I have a NOS '85 Rossin with round Columbus tubes, that I'm almost finished building up and it's a blue to white fade like your Rauler.
Coincidences all?
Yeah, probably.
Cordially,
Mark
Mark Fulton Redwood City California USA
PS: I can only imagine what's involved with threading that rear derailleur cable through the down tube, bottom bracket and chain stay. I had to seek professional help in threading my Rossin's cable through the top tube. I'm only adding this because my friends are constantly suggesting that I seek professional help.