Actually, GP wasn't dropped - GP was changed into PG.
*Unnecessary, off-topic ramblings about the MPAA ratings' history below*
The original ratings, from 1968 to 1970, were:
G (same as today; General Audiences, all ages admitted)
M (Equivalent to the PG rating before PG-13 was introduced; Mature Audiences, all ages admitted)
R (same as today; Restricted Audiences, children under 16 required adult accompaniment)
X (not trademarked like the others; no one under 16 admitted, period)
In 1970, since people kept getting M (Mature Audiences) and R (Restricted Audiences) confused, they changed the M rating to GP. They also raised the admitted ages of R and X films to 17.
So, the second set of ratings, from 1970 to 1972, were:
G (General Audiences; all ages admitted)
GP (Parental Guidance Suggested; all ages admited)
R (Restricted Audiences; under 17 requires accompanying parent or adult guardian)
X (No one under 17 admitted)
By 1971, people were complaining about the GP rating, claiming that it wasn't indicative of the content of the film (it was perceived as too close to a "G" rating, since the G came before the P). During this time, special indicators ("This film may not be suitable for pre-teenagers") were used.
In 1972, the MPAA changed the GP rating to PG in order to address this issue - now, with the P coming first instead of the G, it was different enough from G that people could tell the difference.
1972 to 1984:
G (General Audiences, all ages admitted)
PG (Parental Guidance Suggested - Some Material May Not Be Suitable for Pre-Teenagers; all ages admitted)
R (Restricted Audiences; under 17 requires accompanying parent or adult guardian)
X (No one under 17 admitted)
Then, in 1984, complaints that too many PG-rated films were too violent for such a "soft" rating became overwhelming with the releases of Indiana Jones and the Temple of Doom and Gremlins (both rated PG). Steven Spielberg suggested to then-MPAA president Jack Valenti that they should create an intermediate rating (he suggested PG-14). After conferring with theater owners, PG-13 was unveiled in 1984.
1984 to 1990:
G (General Audiences, all ages admitted)
PG (Parental Guidance Suggested - Some Material May Not Be Suitable for Children; all ages admitted)
PG-13 (Parents are Strongly Cautioned to Give Special Guidance for Attendance of Children Under 13 - Some Material May Be Inappropriate for Young Children; all ages admitted)
R (Restricted; under 17 requires accompanying parent or adult guardian)
X (No one under 17 admitted)
In 1986, the wording of PG-13 was changed to the more succinct "Parents Strongly Cautioned - Some Material May Be Inappropriate for Children Under 13."
Now, all this time, the MPAA hadn't trademarked the X rating. Therefore, it could be used by anybody - and it was, particularly by the porn industry. Because of this, the X rating had a strong "porn stigma" attached to it, and it was damned near impossible to get a film released with an X rating in the United States.
So, in 1990, the MPAA changed the rating to NC-17 and trademarked it.
1990 to present:
G (General Audiences; all ages admitted)
PG (Parental Guidance Suggested; all ages admitted)
PG-13 (Parents Strongly Cautioned; all ages admitted)
R (Restricted; under 17 requires accompanying parent or adult guardian)
NC-17 (No children under 17 admitted)
Before the turn of the millennium, the NC-17 age limit was raised by changing the wording. It currently reads "No children 17 and under admitted," effectively making it "NC-18," without having to change and re-trademark the logo.
And there you have it - the history of the MPAA rating symbols, all in one off-topic post!
*Back on topic*
So anyway, my point is, THX 1138 was always rated PG until the Director's Cut was released - it was just rated PG before the rating was called PG.
SilverWook, could you provide a scan of the lobby card? I've never seen an X rating attached to THX 1138 in any way. Not that I doubt you, I'm just curious.