I've finally got around to shooting this, after a lot of research into it.
Many of you have replied to threads of mine asking about movements etc.. & I believe I've got to the bottom of it, thanks to various helpful people & the OTD archives. There is so much mis-information out there surrounding these beautiful little chronos, and a large number of suspicious examples too, so cheers Jeff for the wealth of info in the archives and catalogues, and all that contributed. Jeff, as always you are welcome to use any pics/info from my blog.

An exact date isn't possible, unless someone out there knows any better?

Anyway, it's a Ref. 701/Dec with a Valjoux 61 movement.
It has the Decimal scale on the outer of the dial. There as a standard 701 (which David D owns/owned), and a 701/Tachy. There is also a 701/Pulso (pulsations per min), but I can only find this in a later catalogue.

The full research is here: http://heuerville.wordpress.com/2011/06/25/heuer-pocket-chronograph-ref-701dec-c1930-38/

If anyone has any further info to add, please let me know.

David D - I used your photos of your standard 701, please let me know if this is OK.

701/Tachy scale version (centre) from the 1938 Heuer catalogue...

Screengrab from the TAG Heuer website history section.. very similar design...

David D's standard 701... note the numbers on the 30 min register stay upright...

Thanks all
Stewart