Wow, sometimes you just "have that feeling", and I had a look around AGAIN and actually found a proof set! Originally this was to be of unlimited mintage and then dropped to 2500; subsequent to that the estimate is less than 50 sets. I think interest had waned in what is essentially a base-metal set and so a rarity was born.
The significance is that there may be other sets of similar scarcity out there but since the RM is loathe to publish numbers of mintage or release, we have only anecdotal evidence. I don't understand why they are not forthcoming....