In the book "IBM's 360 and Early 370 Systems" I found the
following
P-letters related to DOS and OS360. The numbers (84, 123 etc) refers
to the footnote numbers in the book.
84 IBM Programming Announcement, 7 december 1966: System/360 Disk
Operating System, System Release 6, Letter P66-111
123 IBM Programming Announcement, 31 March 1966: IBM Operating
System/360 (Primary Control Program), Letter P66-22
124 IBM Programming Announcement, 29 April 1966: IBM Operating
System/360 (MFT), Letter P66-30
125 IBM Programming Announcement, 7 December 1966: IBM Operating
System/360 Release 8, Letter P66-114
126 IBM Programming Announcement, 30 August 1968: IBM Operating
System/360 Release 15/16, Letter P68-120
127 IBM Programming Announcement, 8 August 1967: IBM Operating
System/360 Release 12, Letter P67-81
185 IBM Programming Announcement, 18 August 1966: IBM Operating
System/360 Release 4 and 5, Letter P66-72