The sequence of
obv. legends and
rev. types under G. III is clear, but how you divide this sequence into issues is up to the individual scholar.
I myself usually divide by changes of
rev. type. As long as the same
rev. types continue, that's the same issue in my book, even if the
obv. legend changes and the date on the
rev. changes say from TR P II to III.
Others might divide my one issue into three or four, making a new issue when the
obv. legend changed and another when the date advanced.
Your
Aequitas coin is from the first
part of the second issue on my scheme. The
attribution you have, 3rd and 4th issue, may be from Jerome
Mairat.
You see why it is essential, when you attribute a coin to an issue, to also name the AUTHOR of the scheme you are following! The division into issues, as you correctly say, is not written on the coins, it's merely a modern construction!
However you number your issues, if you've done it right, you've established the chronological sequence of the coinage of that particular reign. Say there are 15 clear chronological stages in a coinage; everyone will agree what the sequence of those 15 stages was; so it's not terribly important that one scholar may call that 15 issues, while another calls it six issues, a number of which however are divided into several parts.
Attribution to particular workshops, however, is a different matter: it is TOTALLY ARBITRARY, as long as the
officina numbers are not written on the coins! It's only an assumption, not a certainty, that each
rev. type was produced in a different
officina; but granting that, which
officina produced which
type, unless the numbers are written on the coins, is totally unknown!
I can imagine why your
Aequitas type was attributed to
officina 1: because it's alphabetically the first
type, and because the author of your scheme thought it would be nice if the
mint attributed
types to
officinae in alphabetical order. When
officinae numbers begin to appear on coins from
Philip I on, however, we see that alphabetical order was of course NOT followed!
Assigning
officina numbers to unnumbered
types, in other words, is inane and misleading: it pretends we known something that we do NOT know and never can know!