What were the initial and maximum GS ratings for OPM Investigators before 1996?

In 1996, OPM privatized their Investigations division, with the majority of the contract going to a company founded by the former Investigations employees: US Investigations Svcs, Inc. (USIS)

Fast forward nine years, and yours truly is a lowly Field Investigator working for USIS, initially hired at a salary roughly equivalent to the current GS-7 salary (Locality Payment taken into consideration). But being a contractor, and not a true Federal employee, I am not GS rated, which is pretty important for pursuing Federal jobs. If I knew what the span of pay ratings the Field Investigator/Special Investigator position was circa 1996 (or earlier), I would have something more to put on my resume than the fact that my salary nearly coincides with a given GS level. Documentation that the Investigator position was GS-5 through GS-9, for example, would help in resumes and interviews when I would want to say that my current duties are equivalent to those of a GS-7 back when OPM was still Federal.

I’ve tried Googling for old OPM job openings, reports about Investigator pay, etc, and come up dry. I’m hoping that others of the Teeming may be able to find what I could not: What were the GS levels for OPM Investigators before privatization in 1996?

I predict this is far too specific and insignificant a request, and the thread will (justifiably) sink without a positive response. But there are also research hounds on this Board, and I put my faith in them.