It is shown through an analysis of the electron-spin-resonance spectra that the Sn2+(5s2) impurities in KCl can also give rise, after x irradiation above 220 K, to the so-called Sn+(1) centers of the laser-active-type structure. The essential core of this center is a substitutional Sn+(5p1) ion strongly perturbed by an adjoining anion vacancy along the 001 direction. The observed orthorhombic symmetry, with the three crystallographic axes as the main axes, is induced by either one or two weakly perturbing cation vacancies in the neighborhood. Their exact positions are hard to establish and several possible, subtly differing, defect models are proposed. An analysis of the hyperfine interaction of all the np1 (n=4,5,6) centers in KCl is presented, and it is established that these atoms and ions possess large negative unpaired electron-spin densities at their nuclei when they are free or in crystal fields possessing inversion symmetry. The strong odd field component induced by the anion vacancy invariably adds, through s mixing, a substantial positive contribution to this spin density. © 1985 The American Physical Society.