Huw,
There's a forum for Hewlett Packard collectors. On it,
there's a thread discussing Nixie Clocks and sixth post down , a forum member called Paul Berger (Canada) discusses building a clock with an LSI chip, 40 years previously, and also, more excitingly, 'recently'. This was in 2016.
It may be a long shot, but if you can access the forum in some way and then contact this constructor, you may find illumination...