I struggle to see how idlers can be model specific?
The idlers themselves are at varying diameters, as the distance between the ideal chainline and the mounting spindle (on drive side idlers particularly) often vary between models. A lot of terracycle kits include the mounts too, and these are almost all model specific. Some use drive/return on the same spindle, some allow inboard/outboard movement, all seem to vary even within a manufacturer.
I know on this last point, 'cos the 2010 Challenge idlers I've been given to fit to the 'bent won't actually fit my 2009 model Furai.
