(data will be precised as more information becomes available)

The 2X4 brick was introduced in 1949.

The first type (2X4vs01) is illustrated below. The studs are very slightly concave but I have seen examples with very concave surfaces (the concavity results from an excessive thickness of the plastic associated with a cycle too short and/or a lower mold injection pressure than needed, and thus depends on the injection machine used and how it was regulated). When seen from below, the inside corners are rounded. There is not any inscription on the bricks (logo or mold number). The bricks had one or two lateral slots.

NOTE: The slots are molded and not cut (as I thought earlier) and thus the same molding unit always produces the same type of slotted brick.

The next version was introduced in the early 1950s. The main difference lies in the inscription "LEGO" in block letters on the underside.

Seen from below the corners are only slightly rounded (much less than vs01). The studs are slightly concave but as said above, this feature is not molded and thus may vary.

The molds are not numbered but later in its life the surface of the molds was manually scratched with an incisor, so as to inscribe a number in the inside of each part. The blue brick below is number "4" (faintly seen on its right side, near the "O" of "LEGO").

Circa 1954 a new mold was produced. Bricks are still slotted and now had the so-called LEGO "bone" logo inside and a mold number inscribed near the logo (see figure below). Numbers from "1" to "8" were seen.

Seen from below the inside corners are sharp angles. Seen from above, the sides of the studs seem sometimes taller than the top- see the right side of the white brick in the picture below.

This version seemingly co-existed with the two previous versions and 700/x sets mixed together bricks of the three types. The long life of the molds (resulting mostly from relatively low sales) was responsible for this co-existence and also for the variability of bricks from the same mold, particularly in colour and the concavity of studs.

The problem with the concavity of the stud surfaces seen e.g. on the 1X2 bricks called for hollow studs. That feature was introduced in new molds that included the "LEGO" logo on the studs. The fourth version of the 2X4 brick was introduced circa 1956 (see figure below). Studs are now hollow.

This brick occurs (rarely) with slots but is usually seen without.

Mold numbers seen are "1" to "11" and there are also unumbered bricks.

Bricks of this type were certainly sold in 1956-57 in Scandinavia and in Germany but were not seen in sets sold in Portugal.

NOTE: I have seen an image of a hybrid 2X4 brick with this general aspect but slotted and with "LEGO" inscribed as in vs02. This surprising version may have been manufactured out of Denmark and has not been accounted in this list until I can study a few such bricks.

The fifth version of the 2X4 brick is easily distinguished by the underside tubes that connect firmly each brick with the bricks below.

All bricks bear underside the note "Pat. pend:" above and below the central tube. Mold numbers seen are "1" to "20".

This was the first type sold in Portugal from 1957/58.

New molds were introduced in the early 1960s. These are distinguished by the smaller diameter hollow of the studs, correcting a possible fragility of the previous version.

Molds are now identified by a code consisting of a number and a letter, the number being rotated 180º from the position in vs05 (the brick below is "18 E".