A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Jul 15, 11:37 -0700
Francis, you included a link which isn't quite a link, but with a little searching, it gets us there :). Searching their site for "spiral slide rule":
Tennants Auctioneers: Spiral Slide Rule search.
You'll have to click on past sales since there are none currently up for auction. Examples in excellent condition have sold for hundreds of pounds, while examples in "worn" condition sold for £80-90. But those are in the original box. The somewhat worn example that David has seen, with no box, would presumably have an auction value of around £40 or about $50 (USD).