Simon,
What a set up, put my garage to shame.
Here is a photo I took about three years ago of a set of fiffels made by/for a guy called Paul. I can't remember his surname but he had to give up the 1/6th tanks due to ill health. I thought that these maybe the ones you had acquired, but I see subtle differences with the caps.
Here are some pictures of the conversion of my Armortek filters to make the twin drum earlier type. Made from a broom handle, plasticard and P38 body filler!!! No good for the "must be metal" people I'm afraid. Apologies for the quality of some of the pics.
