[KTL]

Here's the factory's explanation from the 74-89 Parts Reference:

Porsche Tech Tip

Poor Starter Engagement:

To improve starter engagement on 87-89 911 Carreras with G50 transmissions, a wider ring gear was installed on 89 model year cars. This new gear was installed in engine numbers 64K 01999 thru 64K 02208, and from 64K 03112. When repairing cars with starter engagement problems outside of the above engine number ranges, the starter ring gear should be checked for damage. If the ring is damaged, install the new wider ring gear part number 950 116 143 01. The starter gear should also be checked for damage.
Important
Make sure the ring gear is installed correctly: Flat surface towards pressure plate. Recessed surface towards bolt head. Torque bolts to 24 Nm/18 lb-ft. See PCNA Technical Bulletin Group 2, Number 8708 for additional info.