Attendance numbers are also not something that can be used to make a fair comparison of the movies from different eras, basically not much different that the inflation adjusted box office numbers.
The inflation adjusted boxoffice numbers are not based on the general inflation or dollar value, they are based on ticket price inflation and estimated attendance. They multiply the attendance number by the latest average ticket price (currently at $7.94) to get the inflation adjusted boxoffice. If there are no attendance numbers available then they create one based on average ticket price of the year the movie was released.
For example the average ticket price in 1982 was $2.94, TWOK boxoffice was $78.9M, resulting an estimated attendance of 26.8 million. Multiplying that by the 2013 average ticket price of $7.94 creates the inflation adjusted boxoffice number of $213.1.