Thanks for the link and the information.
David, I did suggest to my father that the filter goes on before the invercone but he didn't think it did so we didn't try it. He should have known better as he has probably owned most Weston models.
In the 1960's and 1970's when he was a part time wedding photographer, his main employment was as a central heating designer. The Sangamo Weston representative who visited every six months would always bring him a new meter and take the old one away.
(Sangamo also manufactured central heating timers).
Incidently, I too have a spare invercone for a Weston IV or V if anyone needs one