Very interesting thread that took me down a half dozen related and unrelated worm holes. In my very novice opinion, it does seem significantly more likely HPC is the retailer rather than “High Precision <C word of your choice>”
If HPC was added by the retailer it seems very odd that would show up in official UG ads, but then again maybe that is a fitting amount of peculiarity for UG. If it was added at the factory it does seem odd that no Sala reference images contain HPC. A fair amount of examples are not chronographs which would cut down the likelihood of appearing in Sala I suppose.
One area to explore that may help put this “beyond reasonable doubt” (and please do let me know if it already is via another thread or medium), do we know what other brands, if any, Henrique Pfeffer retailed? Do we know what year the shop closed? In particular was it before or after the fall of UG? I suspect with that information there may be other evidence to collect.
HPC is such an obscure designation to put on a dial, my thinking is that perhaps more likely it is replacing what was already there, rather than adding something that was not there......but it's just a guess.