Identify a screw (bolt?) please

[StephenOak]

The chairs were bought new, c. 40 years ago in Suffolk. So probably not for the export market.

That was long ago enough that before asking I did wonder if this might be an imperial screw, but the head is clearly 13mm and is just too big for a 1/2" (12.7mm) spanner. I suppose an Imperial bolt with a metric head is unlikely but not impossible!

11/32" is 8.73mm and the screw is closer is closer to 10mm than that.

 

[Burnerman]

40 years old, and the coach screw is sure to be imperial...say 3/8".
In those days, the spanner sizes were Whitworth which would be a coincidental fit to 13mm.
John

 

[JohnD]

I wonder if the head size is 13.34mm which is 1/4" Whitworth spanner

9mm is almost 3/8"

The hot money is on to it!

 

[Stivino]

3/8” is bang in the middle between 9mm and 10mm. A 9mm spanner is too wee and a 10mm is too big for for a 3/8 hex.
I remember when the Colts (Mitsubishi now) and Datsuns first started to appear in 70’s, nobody had any spanners that fitted them.