Found a solution today ! And I am so happy ...
I bought a "Manuscript" Pen Cap and Barrel with Ink Convertor.
The Ink Convertor fits the Osmiroid sketch nib perfectly. The ink convertor is a very simple (primitive looking) plunger type.
NO other convertors fit ! Not Pelikan / Rotring / Sailor / Faber Castell .... nope none fits....
The Manuscript barrel and cap does not fit the Osmiroid cap nor barrel. Different size.
Anyway, I am so happy... I can see many more years of use with the Osmiroid sketch nib. An excellent nib designed to take India ink or Higgins waterproof ink .... Thats the value of this Osmiroid sketchpen.