The molds for the bricks with split tubes and mold numbers 77 to 97 were revised a few times. I think this was done to improve the mold release of the bricks during production.
Each of these steps makes the brick stick a bit more to the inside half of the mold when the mold halves separate.
Version A
The first bricks (with pat. pend and pat. pend erased) had smooth inner walls.
Version B
Small, triangular cuts are made into the corners of the mold.
Version C
A thin inner “rib” is added inside the long walls.
Version D
The inner rib is now present on all inner walls.
Later bricks used a design similar to version D with inner ribs on all inner walls. On those molds the cut introduced in version B is not used.
(This is a new image of something I described earlier so I can refer to it more easily.)
[Original post and comments on Flickr.]