VMS Help Sys Parameters, RMS_PROLOGUE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    RMS_PROLOGUE specifies the default prologue RMS uses to create
    indexed files. The default value 0 specifies that RMS should
    determine the prologue based on characteristics of the file. A
    value of 2 specifies Prologue 2 or Prologue 1, and 3 specifies
    Prologue 3. The RMS prologues are described in the OpenVMS Record
    Management Services Reference Manual.

    RMS_PROLOGUE is a DYNAMIC parameter.