There seem to be five different molds:
Old-LEGO-logo:
- A (1-14) Pat. pend
- B (1-14) Pat. pend, sometimes erased
- CA plastic, no markings (not shown, non-production?)
M-logo:
- Number below third tube (1-8) Ridge on long side
- Number between short side and tube (1-20) Ridge on short side
All molds have pip on the short side. Since the pins that make the stud hollows were so small, they often broke off, making a filled-in stud or even a peg on later mold runs.
[Original post and comments on Flickr.]