Well, I think this means that if we find initials that are not related to where Sam was, then they indicate the person who worked on the piece with him.
We might have suspected this before, but it seems to be confirmed. I cannot think that the book is inaccurate.
There are other prefixes used. SA and a number for Southern Australia, '74-'79.
VAL for Val St lambert
The Lots Road Studio, '79-'84, used a prefix of "SJH" or "CA to CZ" or "CAC" before a number, while production work there is marked Sam Herman Studio.
A few pieces made at High Wycombe have the prefix HW
and 21st century work is not numbered.
I hope I'm not reproducing too much out of the book for copyright troubles!
I can say I recommend the book very highly indeed.