Q& A finished:

Was copper an original color? No, it's a repaint.

Does the 173 stamp on right top of the seat tube indicate January 1973? No, top of line pro model for year 1973

Does the 204 stamp on left top of seat tube indicate bike 204 for the year? I think so.

The frame has interesting measurements (C-C), 56.5 seat tube, 58 top tube. Explains the super long seat post, and the fact that the spare fork from a 57cm frame is 1/2cm too big.
Well, here are some more detail photos,

Not sure what's with the holes in the cutouts - looks after-market (was drillium-guy here?)

I guess frame number 204


Top of line model for 1973


BB has standard stand-rash


Interesting vent holes in the rear


annnndddd, lookie what color is peeking out from the head tube


love the heart cutouts