A very well used (see that pump cap wear!) and very interesting survivor from late WW1 or just after. An Optimus No.1 made in Austria by GB&R (Gerson, Boehm & Rosenthal), apparently fruits of some sort of deal done between the 2 companies to get round problems caused by the First World War. [Spiritburner has a detailed post on this issue contributed to the Austrian Optimus 100 entry - https://classiccampstoves.com/posts/199510 ] On this No.1 the details of the filler cap, the original early form of 'Optimus' designation on the burner and restriction of mention of GB&R to the pump tube cap I think all favour Ross' licensed product thesis. There is also the very naughty Optimus commandeering of the Primus stove logo in the tank top design! There are no other markings anywhere else on the tank. The rip-off of the Primus logo looks remarkably similar to that which appears on the slightly later and otherwise unattributed 'Sirius' and 'Astra' stoves could this link this same Austrian manufacturer to those too? Not planning to do anything much else to this old timer - it works perfectly well and the wear and patina are part of its history, as is the fantastic cast top plate.