Durifort and Reynolds made tubing in both Imperial (inch) and metric sizes. The seatpost size for metric 7/10 seat tubes is usually 26.4mm. Zeus could have used lighter gage Reynolds 531 seat tubes say, 6/9 0.6mm x 0.9mm wall thickness (20/23 BWG). They would have used a 26.6mm seatpost (or the seat tube could have been reamed out to that size).
Measuring the OD of bike tubing is not always the best indicator of metric or inch size. Last night I measured the tubing on a bunch of bikes (again). I have a 1965 Swiss Tigra bike with metric threads. It has English Reynolds 531 decals so I was curious about the tube sizes (they were metric).
Over the years, I've seen frames with metric threads and inch size tubes plus vice versa. I have a 1974 Gitane Tour de France with metric tubes and British threads. It came to the US from Australia, probably via the UK.
Getting back to tubing diameters, many bikes from the 70s on had thicker coatings of paint. Because of this I usually add about 0.2mm per side when measuring tubes. For example, last night most of the metric tubed frames had 26.4mm top tubes and 28.4mm seat and down tubes. The inch size bikes had ~25.8mm top tubes and ~29mm seat and down tubes.
Older bikes with thinner paint like the Tigra where closer to nominal sizes. Also I measure several points along the tubes and have found differences in the tubing diameters.
On your frame, the red, white and blue Durifort decals on the fork blades were slightly more robust than the water slide Reynolds 531 decals at the top of the seat tubes. A lot of those were damaged or destroyed by bike stands that clamped the seat tube.
Chas. Colerich Oakland, CA USA
Nicholas Brunner wrote:
> The frame could possibly have 531 main tubes. The seller claimed it was
> made of Durifort, and since the steer tube and seatpost (26.6) were
> French, the claim made sense.
>
> Nick Brunner
> Brooklyn, NY