As we discussed recently, the old logo BASF bricks have the mold number inverted while the Bayer bricks have the mold number upright with the exception of number 3. This photo shows how to tell the difference between the BASF and Bayer inverted 3.
The red brick on the left is a regular production brick made in the mold that made the BASF bricks later. The green brick on the right is a Bayer brick.
The most obvious difference is the orientation of the Pat. pend. mark.
[Original post and comments on Flickr.]