#2
The work has been arbitrarily dated. In this case, I think, Harris published his work in three parts from 1776 till 1780. This is probably according to the dates on the work. It means that the first part was published in 1776, the third part in 1780, but it was unknown when the second part was published exactly. Since the date can be important when it comes to priority of names the earliest certain date is given for such works and the date is mentioned between square brackets. Since it is likely that the second part was not published after the third part, but perhaps at the same time as the third part, it is given the same date as the third part, in this case 1780, but given between square brackets.
#3
The square brackets are used if the year of publication or name of the author are esablished
from indirect evidences, say, certainly after 1779 but certainly before 1781; title page contains no year.
Usually this information is useful in big catalogues only, but
redundant elsewhere; as an editor of a faunistic lists, I insist omitting them at all.