I cannot throw more light on the post but I knew there were two interlinears. Not hugely different although I'm sure someone can highlight anything significant.
I have the blue one (later) but not the earlier. As far as I recall the purple one was based on the complete (old green 1961 nwt bible) in style and came out in 1969.
The blue one of 1985 came out with the revision of the 1984 nwt (you know where Heb 1 v 6 is changed from "worship" to "obeisance" of Jesus in the main text) and at the time of the big reference bible.
I guess there would have been tiny subtle differences to support their doctrine plus some extra charts and such to make it more sellable (financially) to the JW's who would have been the main source of income.