I see what you mean. Yes. the
Failmezger book does have a chapter on that.
The short answer is that there is no short answer.
Paraphrasing from
Failmezger: Outside of the letters of the
mintmark itself (including any honorifics - SM - or
officina marks) many of the marks in the
exergue were likely control marks to indicate the minting sequence. I assume this refers to when a particular batch of coins was made - the first batch might have one mark, a batch made on an
anniversary may have a
wreath, etc.
In the
field, he cites several different possibilities, including
officina marks, abbreviated slogans (letters like PR), control/sequence marks, etc. These seem to be fairly speculative, and don't appear to be well understood in many cases.
I don't know of any other sources that discuss this in as much depth though.