Are you referencing the correct manual? The one I have for V7.70 doesn't list any PR objects at all. Just registers and alarm objects. I don't think PR object were supported until the R30iB controllers.
They probably simplified there documents, the ones I got from CRC today for 7.7 and 8.3 had the exact same revision in the PDF. There is a note in it that says positions (7b, 7c, 7d, 7e) are not available before 8.1