Help! Holdsworth seatpost diameter?

[djoptix]

Last year I risked a BIN of £20 on something listed as "road bike" with a grainy photo. It turned out to be what I think is a 1978 Holdsworth Equipe, a bit battered but structurally sound.

I am now resurrecting it as my commuter but I have foolishly thrown out the seatpost that was in it. Googling suggests that some Holdsworths were 27.2 but mine is definitely smaller. The top of the seatpost is a bit misshapen anyway, it measures just over 26mm one way and just over 25mm the other way.

Does anyone know what size it really is, so I can bash something in with impunity? It looks like this, only tattier:

 

[keithglos]

Possibly plain 531 which could be 26.8, butted 531 was usually 23G, plain was 20G, if you can post pics of the details it might be possible to get near the date. The pic you show looks late 1960's, with plain plate ends, a lamp bracket boss and and pump pegs in front of the seat tube.
I sold a lot of Holdsworthy frames in the 60's, mostly the cheaper models.

 

[djoptix]

I'm not sure what you mean by 23g and 20g? Mine does have the lamp boss and the pump mounts in front of the seat tube. When you say "plain plate ends", does that mean the rear dropouts? They could certainly be described as "plain plate", they're nothing fancy.

 

[djoptix]

I tell a lie - the pump bosses are under the top tube.

 

[keithglos]

In the simple olden times when these frames wers made the british seat tube was 1.125 inches outside, and the wall thickness was expressed as "gauge" or G.
Basically similar to standard wire gauge (SWG), the normal 531 plain gauge was 20, or just under .9mm, and butted was 23, or about .635mm.
The later Columbus SL was .9mm and .6mm.
With 531 the difference in internal diameter was just over .5mm, so it is difficult to decide whether you have 26.6 or 26.8, 26.6 would surely go in, and the seat lug clamp should take it, unless very stretched.

Other things I miss: Saddle pillar, fork column, handlebar extension, wired on tyres, (we got rid of clinchers 110 years ago) gearlever, large flange hubs, and quill referred to a type of pedal.

 

[djoptix]

The seat tube is definitely smaller than 26.0.