Why don't you post pictures of the entire gravestones, so we may see the context?
If I had to guess, I would say these are the graves of holy books (siddurim, chumashim, etc.).
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Steve Goldberg
Jerusalem, Israel
Researching:
Sagan/Shagan family from Veliuona (Velon), Lithuania
Goldberg family from Vidukle, Lithuania
Susselovitch/Zuselovitch family from Raseiniai (Rassein), Lithuania

This hasn’t yet been mentioned:  The central symbol for the woman is the candelabrum, representing the lighting of Shabbat candles, traditionally done by the wife. 

the book motif is a common one, representing scholarship. Whether either or both were scholars is unknown. But we are the “people of the book,” after all. 
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David Dubin
Teaneck, NJ

ב״ה

These are absolutely classic gravestone markings.  The letters פ״נ are an abbreviation for פה נקבר - here is buried.    The Star of David is a common symbol of a Jew and particularly a Jewish man.  The candelabrum is a standard symbol for a Jewish woman.  A book on a man’s grave usually symbolizes scholarship; on a woman’s grave, it represents a prayer book and symbolizes piety. 

Yale J. Reisner
Warsaw, Poland
JGFF #913980
<yalereisner@...>