JM- Jan-Mar; AJ April-June; JS July-Sep; OD Oct-Dec. From the early 20's until 1924 the edge print 'Agfa' was printed in 'BOLD' typeface with the top of the A flattened. From 1924 on the type face was thinner and the A had a sharp top. Others have dark patterns at the location of the strips.
Of course, it's easiest if the film tells you what it is.
Starting in the 30's, manufacturers printed or embossed nitrate” or “safety” on to the edges of the film.
Cellulose nitrate was introduced in 1889, and discontinued in 1951.
Cellulose acetates were first produced in the 1920's and are still manufactured as roll film bases.
Kodak stopped making film pack nitrates in 1949, ten years after nitrate sheet films were discontinued.
We often find nitrate film pack mixed between acetate sheet film negatives from the 1940's. On first look, the film base is thinner than sheet film, and the image may be off center.
In our collections, the last Agfa nitrate sheet film was exposed in 1945.
Sheet films can be identified by the notches cut into the upper right corner of the film.
Since 1990, staff have been identifying the film bases, segregating the bases from each other, and keeping them in cold storage.
I started out by identifying the film bases in two large collections.
Cellulose nitrate and acetate films deteriorate at normal room temperatures.