This is a Gibbs-springwire-rope ascender in one of its various forms, though we are not quite sure which model as continuous changes/refinements over the years are difficult to pin down.
Gibbs Products Incorporated are an American company who started making ascenders for caving back in the 1960s and we suspect ours to be a fairly early one, maybe 1970s/80s. Though not a caver, founder member of the SMHC, Mick Tighe, used this one for mountain rescue training back in the 1980s/90s.
If you are interested in these kind of things check out the website,
www.storrick.cnc.net, where you will find hundreds of them.