For the K-like ions ( Z=19± 1 3 , ± 2 3 ) wavelengths for 4s-4p, 5p; 4p-5d; 4p-5s; transition have been calculated by the Dirac-Fock method. The differences between the theoretical Dirac-Fock values and experimental values for known transitions for Z=19–22 exhibit Z-dependent regularities. By interpolation of these differences to Z=19± 1 3 , ± 2 3 the theoretical Dirac-Fock values are further corrected. These values agree very well with values obtained by interpolation of experimental values. A number of transitions are found in the region of visible light which could serve as signatures in an optical search for “quark atoms” i.e. atoms with fractionally charged nuclei.