The Collaborative International Dictionary of English v.0.48:

mutual savings bank \mutual savings bank\ n.
   a state-chartered savings bank owned by its depositors and
   managed by a board of trustees. Abbreviated MSB.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

mutual savings bank
    n 1: a state-chartered savings bank owned by its depositors and
         managed by a board of trustees [syn: mutual savings bank,
         MSB]