A few years ago I contacted Reynolds in regards to this very question. The imformation in regards to tubing thicknesses on my site was reviewed by a Reynolds representative and the information on the gazelle sire was originaly sent to me by the same Reynolds rep and i subsequently forwarded to the proprietor of the Gazelle site. Reynolds' representative said there could be any number of seatpost sizes primarily due to 2 reasons. Tubes could be either Imperial or French diameters and the wall thicknesses could be either conventional DB* or SL. In reality seat tubes were single butted, green label seat tubes were 1.0/0.7 and red label seat tubes were .7/.55. That gives us 4 potential seat post sizes: Meteric w/ green label or red label and Imperial with green label or red label.
On top of that each manufcturer would/could ream out the tubing as needed which in turn opens up the potential for additional seatpost sizes.
In regards to gauge, its my understanding that both Imperial and French tubing have the same wall thickness, the difference is in the overall diameter (gauge).
Its also importnat to never assume that decal language is indicative of the tubes being Metric or Imperial. Decal language is simply that, the language the decal was printed in, it has nothing to do with anything else. The decal pictured on my website should be considered a 'representative' sample.
Jim Ruggieri aka 'miamijim' on the bikeforum
Tampa, FL USA