I don't think that it is necessarily totally an urban legend.
The story of the Duxford Concorde having the modified fuselage frames ties in well with the "Brit " Concorde 002 at Yeovilton allegedy having a similar configuration under the skin -possibly even so far as including the sway brace hardpoints. The "French" prototypes it seems did not have this.
The story could now be fairly easily verified or disproved I would have thought.