You are correct - the id produced by that command changes each time and I am unable to find any support documentation for it. I missed that it was changing when I ran it multiple times yesterday, however I have verified that it did not change the ID viewable in the OSSV Configurator, which remains the original value on my system.
The Install and Admin guide states on page 68-69:
https://library.netapp.com/ecm/ecm_download_file/ECMM1278870
NDMP Host ID
Indicates a unique identifier that the Open Systems SnapVault agent fills automatically. This identifier is used by NDMP-based management applications to identify the primary storage system. You must not modify this field.
So if these hosts do not yet have working SnapVault relationships I recommend reinstalling OSSV on them to ensure unique host ids.
Kevin