The Chinese text says these are approximate cartesian coordinates. The document I found associated with the these coordinates (likely the same one Yi found) is the border delimitation document supplementary to the bilateral treaty between PRC and Russia on the Sino-Russian border. The coordinates should be a border marker on Bolshoy Ussuriysky Island. The delimitation and demarcation processes involve having a joint committee that determine the geodetic reference system and datum, and would be noted on the actual maps drawn (but presumably these aren't made publicly available, but some sort of grid system like UTM.) Perhaps a creative investigative work would be to see if you can see any markers on high resolution satellite imagery. A fun question - please keep us posted!