Catalyzes the hydrolysis of the ring A acetate delta-lactone of cobalt-precorrin-5A resulting in the loss of the C-20 carbon and its attached methyl group in the form of acetaldehyde. Co-precorrin-5A + H2O = acetaldehyde + Co-precorrin-5B + H(+) Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 5/10. Belongs to the CbiG family.