The secret history of Twin peaks
The crime scene where the archival dossier featured in The Secret History was found I think as to be after the events shown in the series due to Tamara Preston reading the documents and writing things in it that in the series itself she has no awareness of. This would make sense in that Gordan Cole would only allow her to read over the documents once she has experienced what she is experiencing in the series and being inducted into the Blue Rose group.
If I recall, the way she (TP) writes her notes it suggests she doesn't know anything about Briggs or Cooper.
If we are going to assume some element of link to the series, I'm betting my money that the dossier was found before the series takes place. Almost like an induction into the history / background, and to test her calibre for potential Blue Rose candidacy.
Bill Hastings told Tammy he was 43. His ID had his birthday as 8/15/1973. That would mean the interrogation had to occur on 9/29/2016. The dossier was found and examined by Tammy in the Summer of 2016, per the Secret History. Also, going through the dossier seems to be some kind of test put to her, she seems very skeptical at first about strange stuff, etc. So, it makes more sense to think that this was a test before bringing her on to Blue Rose, as opposed to vice versa, I think.
It would seem they aren't going to mention this explicitly on the show, though, and there is some wonkiness about time (see: Wally Brando, e.g.). My guess is we will only find out more about this in the Final Dossier.