Another example of "Coordinates without CRS are meaningless numbers"
https://miprincipado.asturias.es/bopa/2011/06/15/2011-11563.pdf
(Official document - "BOLETÍN OFICIAL DEL PRINCIPADO DE ASTURIAS")
Which CRS should I use with these coordinates?
That region in Spain, Asturias, uses UTM 29 on the west, and UTM 30 on the east.
(of course those coordinates only make sense in one of them... but I had to try/guess)