Determining a poster's age
a.) Pre 1945
Few movie posters survived WW2. So pre-war material is very rare. The artworks on these early posters are often rather simple illustrations.
It is not easy to determine a poster's age if the film was released prior to 1966 and many films were not released in Japan in the same year they were in US.
The best way to determine the release of a poster from this period is to use one of the books that list many of the Chirashi (flyers) that were released in Japan. Since the year a flyer was available is also the year of the film’s release, one can determine the age of a poster. One series of books is called "Chirashi Zenshou" (=Complete Works of Chirashi) of which five volumes have been released so far (1945-69, 1970-79, 1980-89, 1990-95, 1995-99).
There are limitations to this however:
a.) Only non-Japanese movies are listed
b.) Film titles are in Japanese, so a non-Japanese speaker has to guess the film's title from the pictures on the flyer,
c.) While a large percentage of released films is in this book, not all movies were/are advertised with Chirashi and the ones that were not are missing.
In late 1965, a ratings code was introduced in Japan (with INVASION OF THE ASTRO MONSTER, released in December 1965, probably being among the first films to be rated) . Since then the vast majority of movie posters bear a stamp (called "Eirin Mark"), indicating the year they were rated. The stamp consists of two Japanese characters (for “Ei”= picture, and “Rin” = code) and a 5-digit number. The stamp is usually at the bottom right or left of a poster. The first two digits of this number stand for the year the rating was given.
This could be very convenient, if it weren't for some studios that do not use the Western system and count years in the traditional Japanese way instead.
This traditional system is linked to the reigning emperors and every time a new emperor is inaugurated, a new era begins. Each era is given a specific name and starts with year 1. The eras important for collectors are Showa (1926-88) and Heisei (1989-now).
Converting works best by using Showa year 45 (=1970) as a benchmark and then adding or subtracting years accordingly.