Mold pip on stud.
They feel and sound different. They also appear to be quite a bit lighter than normal.
Clutch power is not good.
Mold markings “5” “55” on most of them.
I just checked all the bricks and most of them have the exact markings above, but I also found some with “01” above the middle tube.
Those bricks also have the numbers 3, 4, 6 or 7 underneath the left tube.
The bricks without the “01” mark only have “5” underneath the left tube.
Forgot to say that I did not sort out these bricks from other normal red 2×4 bricks. Absolutely all the bricks I recieved were like this. So it must be something weird going on with the mold or plastic…