I think it was a South American idea or design from 1940's but I too believe the first built in the world was at NPL and easy to remember as 1955 as year I was born.Nor do I, but I imagine that there were probably very few other accessible atomic clocks? I have a feeling that the vey first may have been the one at NPL, which I think appeared at some point in the1950s?
IIUC from an IET lecture at Savoy Place the first to be used after NPL was bespoke made by HP for use at Greenwich in early half of 1960's and following several clones of it, it became available as a standard model. However he also said; history shows it was already a commercial product when Greenwich started acquiring them later in 1960's.
From that I'll assume they probably made a prototype for field testing?
