Lay the computer on its side so that the arrow on the bottom of it points up.
Open the cover: Facing the back of the computer, press the release button with one hand while pulling up on the top of the cover with the other hand. Press the release button on the left side of the computer with one hand while pulling up on the top of the cover with the other hand. Hold the bottom of the computer with one hand, and then pull open the cover with the other hand
Having done a google search for the T600 it would appear that this case was used by dell for some time and is identical to a dimension 386 I had some years ago. If you look at the back you should see a thumb wheel in the centre on the the left side panel. Unscrew this as far as it will go and with the machine laid on its side push the two lugs you mention towards each other and at the same time pull the entire side panel backwards away from the machine. It will only move a short distance and then lift it clear of the cabinet. If it is the same as mine it does sound complicated but once you have done it you will find it very easy the next time.
Hope this is of some use. - Good luck
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