In 1973 LEGO introduced a new version of the brick. (Drawing date according to LEGO vs Tyco, introduction into sets might have been a bit later). On these bricks the mold pip moved from the short side to one of the studs. The flowrib (“centerline”) in the top of the brick was no longer needed. This brick also introduced several material-saving changes. The walls were made thinner with vertical ridges to connect with the studs and the tubes were split. The date “1973” matched the 1973 oil crisis so a sudden increase in material cost might have been the driving force behind the new design.
Looking at my bricks with split tubes I have found a few versions:
- Mold X, patent pending. I found the position numbers 1,7,9,14 and 22.
Nr 77 to 96 (I am only missing 82 and 94), patent pending and erased. Below the mold number is a position number ranging 1 to 6. I also found a brick without numbers (void) that fits this description, so that is probably an unnumbered brick within one of these series. Early versions of this brick do not have inner ribs on the walls, later versions do. Initially there is only a small triangle in the corners, after that a rib on the long wall is added. Later a rib on the short wall is added also. These might have been a modification to improve the mold release.
- Nr 97 and 98. No patent pending. Internal rib on the long and short wall without the small corner triangle. The mold position number has two digits (found 03, 04, 05)
- Bricks with the mold number inside the center tubes. In one of the stud holes is a letter (A to G). The letter and the number do always match eachother, so there are no different letters for different mold positions. Internal rib is full length on the long and short wall without the small corner triangle. The bricks have a “center line trace” where a flowrib used to be. I found the numbers 007 (D), 008 (H), 009 (A), 012 (F), 013 (C), 015 (D), 016 (H), 031 (D).
- Bricks with the mold number inside the center tubes. In one of the stud holes is a letter (A to G). The bricks do not have the “center line trace”. The letter and the number do always match eachother, so there are no different letters for different mold positions. Internal rib on the long and short wall between the outer ridges. I found the numbers 018 (B), 023 (G)
[Original post and comments on Flickr.]