just reinforcing this. Book II RPSI is like ISR book is to Oldenburg...an actual lower book. Just like some folks dont care if they get an ISR brand instead of an Olenburg brand, some RPSI folks don't care if they are in Book I or II, and yes they do have a different brand. The Book II has a half brand.
Just to clarify - Oldenburg (as in OHBS/GOV) doesn't have anything to do with ISR, so you should probably have been more specific and said "Oldenburg NA".
And I thought "ISR" was not a "lower book" of Oldenburg NA, it was a completely different registry. So horses that don't qualify for Oldenburg NA are put in the ISR registry instead.