> for example: "R: [ACS 78-13] Offer", "R: [ACS 79-13] Offer", etc..
I am guessing that the stuff within the [square brackets] is ignored.
These days, square brackets are often used by email lists. Thus, if a
message was sent to two or more email lists with otherwise the same
subject, Gmail would ignore the list names and recognize that they
appear to be the same topic, and would group them into the same
conversation.
Unfortunately, as email recipients, we Gmail users have no control
over how this works, other than to enable/disable conversation view.
As Zack suggests, if you can control the subject line, you might be
able to change that.
Andy