It appears that you have a pump type cap normally found on a smaller backpacking stove on the fuel tank. It will accept a mini or midi pump for improved performance. This cap is redundant in this case but would serve well on an Opti 8r or Svea 123.
can´t see any difference to this: http://www.optimusstoves.com/seen/o...connect/optimus-outdoor-kocher/optimus-hiker/
I am quite certain this is an SRV, designed to open at a certain pressure (when things get to hot). Best regards, Wim
Note however that Optimus had gone to a more standard hex rather than the pentagon hole for the cap SRV.