MQMD_* (Message descriptor structure)

See:

MQMD_STRUC_ID "MDbb"
MQMD_STRUC_ID_ARRAY 'M','D','b','b'
MQMD_VERSION_1 1 X'00000001'
MQMD_VERSION_2 2 X'00000002'
MQMD_CURRENT_VERSION 2 X'00000002'