I am grateful to a Monsieur Gerard Demaison for virtually nothing except this piece of information.
The FT that stood outside the Renault offices at Boulogne-Billancourt fell into serious disrepair, with the idlers rotted away. When it was restored, the idlers were replaced by a French company that specialises in repro wooden wheels. Photos and commentary (conveniently in English) here:
(Nb: If I seem ungracious toward M. Demaison, it is because, amongst a number of other things, he previously insisted for quite some time, on both English and French Wikipedia, that the FT never had wooden idlers, and became quite abusive when apprised otherwise.)
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"Sometimes things that are not true are included in Wikipedia. While at first glance that may appear like a very great problem for Wikipedia, in reality is it not. In fact, it's a good thing." - Wikipedia.
When I visited the Weald foundations FTs I was shown the newly restored Idlers, interesting to note that a bit of horse and cart technology incorporated in the FTs design.