1932 T. E. Herbert & W. S. Procter (ed. 2) I. xiv. 614 Lines terminating on a P.M.B.X. are connected to consecutive jacks.
1975 68 61/2 The PMBX No. 4 was originally designed to meet the requirements of single-position installations of up to 160 extensions.
1986 G. Langley (ed. 2) xvi. 44 Boards like this were called PMBXs, Private Manual Branch Exchanges.