Jeff,
the parts book calls it a "spring-bolt lock". It keeps the rear part (with the window) folded together. The bolt can be turned in or out to increase/decrease the tension on the lock. My spare parts list Edition C [10126] shows that it was used on the 230SL until VIN 019020 and then with about 42 exceptions disappeared starting with VIN 019359, however, the first two 250SL also had it as Naj noted.
Left p/n 113 770 0329; right p/n 113 770 0429.