I've got some collets lying around that are similar to 5C in conception but different dimensions. The main shank is 27mm diameter (it could be a bizzare imperial dimension but I suspect it's metric because the external closing thread is 1.5mm pitch.). They are 109 mm total length, 95mm along the parallel shank (including thread) and the head is 35mm in diameter.
All suggestions welcomed. They don't appear to be in Machinery's Handbook but the sight of a metric thread almost guarantees exclusion. They have the "CC" mark of Crawfords Collets but their web site has disappeared - I suppose that if anyone has a copy of their full manual they could be found quite easily.
Thanks
Charles