This marker is relatively young. It was placed in 1997 after a Spanish engineer had discovered that the borderline between bm420 and 421 was presented wrongly on the maps and didn’t follow the real watershed. See this page for the whole story.
In 2018 Corinne Gourgeonnet visited bm420bis and found both parts neatly against each other:
Now we are four years later. Corinne Gourgeonnet completed her full range of esfr-bordermarkers last year but can’t forget the esfr-bordermarkers. While walking the 5-days cross-border Pass’Aran-trail , she couldn’t help to revisit bm420bis.
And as you see: the upper half has now disappeared, probably tossed down the mountainside.
A famous quote from a famous dutch poet: anything of value is vulnerable.