Panasonic SJ-MJ55GH Service Manual page 17

<MD cartridge type>
[How to open the shutter]
1. Insert a thin and hard card to the crevice between laser power
meter, and pull a card in the direction of an arrow (1).
2. A stopper moves in the direction of an arrow (2).
3. Move the shutter in the direction of an arrow (3) and the shutter
opens.
Caution:
Please be sure to shut the shutter after the adjustment end.
Fig. 8
Fig. 9