Multifrequency electron-spin-resonance (ESR) study has revealed a nontrigonal Ge dangling bond (DB)-type interface defect in SiO2/(100)GexSi1-x/SiO2/Si heterostructures grown by the condensation method. The center, exhibiting monoclinic-I (C-2v) symmetry, with principal g values g(1)=2.0338 +/- 0.0003, g(2)=2.038 6 +/- 0.000 6, and g(3)=2.005 4 and lowest g value (DB) direction 24 +/- 2 degrees off a < 111 > direction toward the  interface normal, is observed in maximum densities for x similar to 0.7, the signal disappearing for x <= 0.45 and x >= 0.93. Neither Si P-b type nor trigonal Ge dangling bond defects is observed, enabling unobscured spectral analysis. Based on its ESR parameters, including g matrix and symmetry, it is suggested to concern a Ge P-b1-type center, that is, not a trigonal basic Ge P-b(0)-type center (Ge-3 equivalent to Ge-center dot), thus exposing a unique interface mismatch healing as function of substrate Ge fraction. Its properties are discussed within the context of the thus far elusive role of interfacial Ge DB defects in Ge insulator structures, encompassing theoretical inferences.