LEGO logos on both studs not parallel (this one is the only version of the brick in which the logos are not parallel).

The LEGO logos on the studs upwards when the brick is is the position as on the side figure.

The studs are not hollowed and because of this, when the molding cycle is too swift the upper stud surface caves slightly in. And so, in some of the earlier editions the surface is slightly concave.

Mold number inside ("1" to "6" were seen).

Lego logo on the studs as above.

The studs are now hollow from the inside, so as to allow for a faster molding cycle. The hollow has a diameter of circa 3.0 mm.

Mold number inside ("4" to "24" seen).

Lego logo on the studs upside down when the brick is is the position as on the side figure. .

Studs with a 3.0 mm hollow, as above.

Mold number inside ("1" to "3").

As previous version but the thickness of the stud wall has been increased (the stud hollow has a smaller diameter than in versions Vs02 and 03 above). This correction was also introduced in other bricks (e.g. the 2X4).

Mold codes are now made up of a letter and a number (the red brick shown is C2).

The third version above is the last that will interest us. However, the side figure illustrates two newer versions with the "LEGO" logo on the studs in two different positions and a lettering type that is taller and somewhat thinner.

Mold codes made up of a number and a letter.