Sarcophagus (probably, of a royal family member). The sign tells you what prayers to read when you are at the head of the sarcophagus, or when you are at its sides.
<a href="//commons.wikimedia.org/wiki/User:Vmenkov" title="User:Vmenkov">User:Vmenkov</a>
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<span class="int-own-work" lang="en">Own work</span>