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First bi-tube frame I've seen with a bottle boss and I do tend to keep an eye out for this type.
It's quite well detailed, not only are the bosses angled to keep the bottle cage centered in the frame but the two seat tubes are angled in a way to give space for the bottle cage - one leg brased slightly in front of the other at the BB...
...TBH the builder could have just had one too many with lunch that day hence why the tubes arent brased adjacent to each other.
Pretty much finished this, just missing a ferrule and it seems the derailleur hanger needs a bit of a tweak.