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.
The first bricks (with pat. pend and pat. pend erased) had smooth inner walls.
Small, triangular cuts are made into the corners of the mold.
A thin inner “rib” is added inside the long walls.
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.]