The BMW R12 engine block is made up as two pieces. When it was at the factory it was bored as one unit. This made sure of a perfect alignment fitting on the crank and bearing carriers from front to rear. When this was done by BMW they marked the case halves to ensure the assembly of the motor was the original two sections. This marking was done by the same numbers on each case halve. From the photos below you can see this marking. Here you can see 3 examples of different engine blocks.