HP 16550A User Reference page 253

I
Installing and Removing Cards
Removing Cards from the HP 16500B/16501A
To remove cards from the frame, you must start with the card in the top slot
(slot A in the HP 16500B or slot Fin the HP 16501A) of the frame in which
you are changing cards. Remove the next card down. Proceed in this
manner until you get to the card you need. To remove the cards from their
slots:
1
Loosen the thumb screws on either side of the endplate of the card
until the thumb screws are free from the frame. If two cards need to
be removed together, loosen the screws from both cards before trying
to remove the cards.
2
Gently, but firmly, pull on the heads of the thumb screws and slide
the cards out.
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