The “Senechalle Counter” was likely never an actual product which could be bought and used by a prospective measurer. It exists as the subject of an article by mathematician David C. Senechalle. The article describes, in detail, how one can purchase various components, assemble them, mark the front wheel, and use the resulting assembly to measure race courses.
The parts described in the article are likely unavailable, but it’s a nice how-to article.
The two page article is entitled “The Senechalle Assembly – a Cable-Driven Bicycle Odometer by David C. Senechalle” and appears to have been typed by Ted Corbitt in the mid 1970’s.
A Google search for David Senechalle produces several references to his existence as a mathematician and runner.
This article, and others, is now online. See http://www.runscore.com/coursemeasurement/