That seems reasonable – kilometre point. But you're not sure.
I have no doubt at all that
@Pelegrin is 100% correct and that P.K. means
Punto Kilométrico. It's a "thing" in Spain and France, according to Wikipedia. And just like milestones can be "placed at intervals of one mile or occasionally, parts of a mile", so can P.K. stones.
The whole point of the costly exercise of replacing the Camino waymarkers in Galicia during the past few years was to make them uniform throughout Galicia. As far as I can tell, today's mojones on the Camino Portugués have exactly the same design as the mojones on the Camino Francés.
Both decimal commas and decimals points can be used in Spain although it is recommended to use the decimal comma. The new Galician mojones make use of the decimal comma.
I find the three decimal digits on the new Galician waymarkers ridiculous. However, my reason is not that an integer or whole number is better. I find it ridiculous because it states (or pretends to state) a level of accuracy down to metre level that makes little sense for long-distance walkers. It is a useless piece of information, and I find it even distracting. One digit after the comma would be more than sufficient.
As far as distances go, they are just distances. They don't change whether you measure them in miles, in kilometres, in any of the many medieval leagues of any of the countries that sent pilgrims to Santiago, or in Roman miles, or in millimetres. As such, a mojón with km49,995 is just as good as a mojón with km50 that is placed five metres further back on the way. Their location is more important than what's written on them because their main function is to tell the walkers where to go or to confirm that they are still on the right track. So it is more than sensible that the administration did not make a fuss and did not try to figure out locations that resulted in "nice" numbers.