Her's an interesting stove :

2196062109

It says primus AND Optimus on it. As far as i know this is done on stoves that were made by Optimus under the Primus trade mark when Optimus took over Primus.

This is also why you sometimes get Primus 8R's (right?)

So this stove can't be WWI, because Optimus took over much later?

I presume from the look of that picture, the top ring is one of the cast iron ones,

When did they replace the cast ring wth the steel one ?

In fact it is only the filler cap & pump collar that are marked Optimus. This is a genuine early Primus with some non-original but old replacement parts. A nice buy for someone.
However I think that lack of fuel & the noise these make would not make it the ideal stove for use in a WWI trench.

Primus first announced the new pressed steel tops in their catalogue No. 831 (early 1920's). The pressed steel top (R880/1 - complete with its own patent number) was announced in the 1920's catalogue as an improvement on the older cast-iron, which was stil offered as an alternative (R 578/9). It was lighter, considered to be stronger (especially since it fastened to the tripod with integral clips) and not prone (as were the older cast-iron top rings) to cracking or shattering. Basically, cast-iron rings for the standard domestic and larger collapsible stoves were made from the beginning through the 1920's (Optimus "cloned" the Primus pressed steel tops about 1927). Cast-iron rings of standard size remained in the Primus catalogue for a few select models only into the mid 1930's. Cast-iron tops for the Primus/Optimus Nos. 2 and 3 persisted right up to the end of production in 1996. Many Optimus Nos. 100 and 45 were supplied with fairly slender, engraved cast-iron rings. Often these had the words: Made in Sweden etched in the underside. For many years, Condrup of London also supplied cast-iron rings for Primus (marked "Con for Primus"). Primus produced single burner ranges with integral cast-iron frames up to the mid 1920's, the Nos. 120 and 121 (and their silent burner equivalent, No. 124).

For domestic use I have always liked the cast tops better than their pressed steel equivalents - I really have a soft spot for that turn of the century cast-iron aesthetic. Check the catalogues (including Radius, Svea and Primus) on this site for further information. Because they were brittle and subject to shattering if dropped, many cast-iron tops have disappeared over time. However, for me, they are absolutely essential to the "original" world of the Primus stove.

And, on the very first Primus stoves, the cast-iron ring incorporated the flame spreader for the roarer style burner - perhaps Ross has a picture of this early prototype for posting.

Thanks, very informative.