This is not what I thought it was when I first saw it! But I did realise that the leg sockets were not of a type that I recognised from any external-NRV Optimus that I could recall. The incuse markings on the filler cap and pressure release screw also promised an early date. The fact that the, 2 pint size, tank then turned out to be marked 'Optimus 100' was the biggest surprise of all. There is no illustration that I can find in any Optimus catalogue currently in the reference library. The earlist of the catalogues available is dated 1911, so this stove must pre-date that. The Optimus v Primus patent case that led to Optimus adopt its own externally accessed NRV system was in 1906. So best guess at a date for this stove is somewhere in period 1906-1910.