Sony MSW-900P Operation Manual page 264

Attaching the battery pack
1
Press the battery pack against the back of the camcorder, aligning
the side line of the battery pack with the line on the camcorder.
5
2
Slide the battery pack down until its "LOCK" arrow points at the
matching line on the camcorder.
5-2
Chapter 5 Setting Up the Camcorder
Back of camcorder
L O C K
BP-L60A
Align these lines.
"LOCK" arrow
Matching line on
the camcorder