Bifunctional enzyme that catalyzes the methylation of C-15 in cobalt-precorrin-6B followed by the decarboxylation of C-12 to form cobalt-precorrin-7, and then methylates cobalt-precorrin-7 at C-5 to form cobalt-precorrin-8. Co-precorrin-7 + S-adenosyl-L-methionine = Co-precorrin-8X + H(+) + S-adenosyl-L-homocysteine Co-precorrin-6B + S-adenosyl-L-methionine = Co-precorrin-7 + CO2 + S-adenosyl-L-homocysteine Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 8/10. In the N-terminal section; belongs to the precorrin methyltransferase family. In the C-terminal section; belongs to the methyltransferase superfamily. Bacterial-type CbiT family.