As per the request of interested parties I'm exhibiting the following.

Below are the rules for the Press proper (not the clean part) taken directly, word for word, from the 1970 AAU Weightlifting Committee Rule Book. Keep in mind that the clean was also part of this lift, and infractions there sometimes resulted in red lights or "no lift'" decisions for the entire lift. Also note below, I've put extra comments in italics.
From Page 34: The Press proper

At the referee's signal the bar shall be lifted until the arms are completely extended vertically above the head*, without any jerk, without stopping, without bending the legs, without exaggerated lean back of the trunk*, without displacement or movement of the feet. Remain in this final position motionless, until the referee's signal to replace the bar on the platform.

*Note: Although it wasn't addressed in the rules, if a lifter happened to have had a lockout problem due to poor shoulder or elbow flexibility, the referees would pass a lift that wasn't fully locked out, provided the lifter pointed out his disability prior to the contest.

* Another Note: In 1956 the wording "without exaggerated lean back of the trunk" was
changed from "During the whole execution of the second part, the Press proper, the body
and head of the lifter must remain constantly in a vertical position. Any deviation of the body or head from the vertical position must be seen as an incorrect movement".

As you can see, the old wording was very strict. The problem the IWF had was that the
Russians were passing so many bad lifts behind the iron curtain that it got out of the IWF's control. In short, they just capitulated! It was easier to loosen up on the rules rather than argue. IMO, the IWF should have stood up for what was right. That wording change was the beginning of the end of the Press.

From Page 37: Incorrect movements for the Press.

1. Cleaning in several movements.
(using a continental to rack the bar)

2. Starting before the referee's signal.

3. Bending of the legs, however slight, equally at the start or during the movement of extending the arms.

4. Flexing the arms after the referee's signal.
(This was known as a "double start" and doing so caused the bar to drop downward slightly. I was red lighted for this myself in one meet and I didn't realize I was doing it. However, the head referee called me over and was kind enough to point it out to me.)

5. Bending the trunk by flexion or extension.
(This means no hunching down. Alexev did this all the time. And this rule was not enforced. Probably one of the main reasons the press deteriorated so much.)

6. Exaggerated lean back of the trunk under the bar.
(Another rule that was not enforced that contributed to the demise of the press.)

7. Uneven extension of the arms.

8. Pausing during the extension of the arms.

9. Incomplete extension of the arms.
(This was allowed if the lifter had a naturally poor lockout that prevented him from fully extending his arms.)

10. Rotation of the trunk.

11. Moving the feet.

12. Raising the toes or heels during the press.

13. Replacing the bar to the platform before the referee's signal.

Details concerning refereeing the Two Hand Clean and Press.

1. The clap for the press.... Wait for the lifter to be absolutely motionless and balanced. If the latter moves the feet after the start without extension of the arms, it is "No Lift" and there is no case for giving a new start, as has often been stated.

2. Details on what is motionless after the Clean....The referee's signal shall be given as soon as the lifter becomes absolutely motionless in all parts of his body.

3. Difference of interpretation.... This can only arise in the slight lean back of the trunk and can only be conceived in a limited manner. This "slight lean back" of the body permitted by the rules, consists of a slight swaying backwards to allow the lifter to pass from the "dead point" to the end of the lift, with the arms extended vertically, and not of an almost complete "lay back"
(What a crock of double talk this was!!! This is very similar to the "subjective judgment calls" on the Squat, that exist in different Powerlifting organizations today.)

4. Separation of the the discretion of the lifter.
(From what Bob Hoffman once told me, in the beginning, the lifters had to stand with their feet together like a soldier at attention. Thus the term "military press". They then loosened up on this rule, I don't know exactly when it was, and said the lifters could have their feet apart, but not wider than shoulder width. Finally, I don't know the exact time, lifters were allowed to stand as wide as they wanted.)

NOTE: Bob Hoffman was the founder of the York Barbell Company, and is manytimes referred to as the "father of weightlifting".

5. Position of the head.... at the discretion of the lifter.

Happy Pressing, JVA