Catalyzes the insertion of Co(2+) into sirohydrochlorin as part of the anaerobic pathway to cobalamin biosynthesis. Co-sirohydrochlorin + 2 H(+) = Co(2+) + sirohydrochlorin Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 1/10. Homotetramer; dimer of dimers. Belongs to the CbiX family. CbiXS subfamily.