See here for an overview of all Minitalia 2×4 bricks.
Material: ? (The plastic is cheaper than regular ABS and the Minitalia bricks often show much wear.)
Inner tubes: X support
Flowrib / center line inside top surface: Yes.
Mold pip: on the short side.
Mold numbers: A in small font size (a different mold with a larger font size A could also exist, but it has not yet been found).
Mold position numbers: 1 to 24. Usually machine engraved, but scratched on brick number 2.
Inner ribs: Full length rib near the top of the brick on both the short and long sides.
Intellectual property protection: “Pat. pend” and “Pat pend”, depending on mold position number. I have not found bricks with the Pat pend mark erased/obscured.
Colors known: White, Red, Blue. (Yellow, Black and Transparent might exist also.)
- No logo or brand name is shown.
- The studs are hollow on the top, there are no holes on the inside of the brick.
- These were the first thin-walled bricks. I believe the thin wall, the split tube and the X-support are all attempts to reduce the amount of plastic used in order to reduce cost as much as possible. I suspect the two types of support were used to test these different options for possible use in regular LEGO bricks later.
[Original post and comments on Flickr.]