IBM B50 - Manual (Page 110)

6. Push down on the memory module until the locking tabs lock the memory
module into the connector. (Do not attempt to move the locking tabs yourself.
They lock automatically when you have fully inserted the memory module.)
Attention: Inserting the memory module at an angle may cause damage.
7. Inspect each memory module and ensure that the module is fully inserted into
the socket.
4-18 7046 Model B50 User's Guide