Exoglycosidase that cleaves the single beta-linked mannose residue from the non-reducing end of beta-mannosidic oligosaccharides of various complexity and length. Prefers mannobiose over mannotriose and has no activity against polymeric mannan. Is also severely restricted by galactosyl substitutions at the +1 subsite. Releases the terminal mannose residue from mannobiose, mannotriose and galactosyl-mannotriose (GM3), but not from galactosyl-mannobiose (GM2) or di-galactosyl-mannopentaose (G2M5). Hydrolysis of terminal, non-reducing beta-D-mannose residues in beta-D-mannosides. kcat is 79.2 min(-1) with p-nitrophenyl-beta-mannopyranoside as substrate. Optimum pH is 6. Active from pH 5 to 8. Optimum temperature is 37 degrees Celsius (at pH 6). Glycan metabolism; N-glycan degradation. In contrast to clade A beta-mannosidases, which are likely secreted, clade B proteins appear to be intracellular. Belongs to the glycosyl hydrolase 2 family. Beta-mannosidase B subfamily.