Welcom to the forum.
If I understand what has been discussed at the various Vintage Airstream seminars and discussions that I have attended, your serial number:
Probably decodes as
I (my guess it that the first digit may be a letter rather than a "1") for International (trim package)
27 = 27' Length (would correspond to Overlanders of the late 1960s through the 1970s).
T = Center Twin Bed floorplan
9 = last digit of model year which would point to 1969 (IMHO)
J = Jackson Center, Ohio (plant where produced)
389 = sequential production number
I am more familiar with mid-1960s serial numbers, but I do think that the serial number points to a 1969 Overlander International with Center Twins.