What is a Series 7 adapter exactly?

Series 7 rings are intended to hold that particular series size filter to a lens. If they fit a WA or TEL conversion lens it's by coincidence.

Look here for an explanation of series filters. The screw thread diameter you want is probably 2 1/8", which is very close to 54mm. I don't know what the thread pitch is. This would actually correspond to the Series VII retaining ring or hood.

If they fit a WA or TEL conversion lens it's by coincidence.

Not necessarily. They may well have made the conversion lens to attach by a Series VII thread on the front of the camera lens.

(Fine print: some camera lenses were like this, although I have no personal experience of it. They had a series thread right on the lens, so you could drop on a series filter, and then screw a hood or retainer right on the lens.)

[quote]I know it converts a "series 7" thread to a specified filter thread e.g. Series 7 to 58mm adapter ring etc. But what size and type is the series 7 thread?[/quote]
A series VII filter (or V or whatever) has no thread. Itś a drop-in thatś held in place by a retaining ring that screws into the adapter.

Thanks all. It sounds as though I need to get a stock of some series 7 rings then. I was hoping the series 7 thread was a standard size thread. Never mind.

You can buy many different size adapters that will adapt to series 7. Here are some on Ebay. (link)

This ought to be somewhat helpful: http://randomphoto.blogspot.com/2007/04/serious-about-series.html

Michael, a small point, but if you click on "cameras..." in the eBay box to the right of "series 7 filters" it avoids the similarly named car parts.

There is a partial explanation of series filters in general at http://www.marriottworld.com/pieces/pieces21.htm of which the main part is:

Series filters are glass or plastic mounted in a metal or plastic rim. There is no screw thread or other mounting; instead, the filter is designed to be held in place by a retaining ring. This retaining ring would either be part of the lens or be part of a lens hood/filter holder.
Series filters are usually referred to using Roman numerals (except Series 4.5).