The earliest bricks with tubes used only a number on the short side. There are several very similar molds like that, and often they are difficult to separate.
Other early CA bricks with tubes include versions with letters only, with scratched marks, completely without marks and the sideways logo bricks.
Next came a lot of bricks with a mold identification that consisted of a mold letter + position number. This image provides an overview of these bricks, as far as I have been able to find them.
Initially they were made of CA plastic, but several of the molds remained in use during the transition to ABS and can be found in both materials. The earliest bricks had a mold pip on the bottom edge (usually one of the corners, sometimes centered), later bricks had a pip just above the middle on the short side. Usually the pip is centered, but different positions can be found on the Samsonite bricks.
[Original post and comments on Flickr.]