what is the meaning of a note that looks like a rhombus in a contrabass quartet arrangement?

[Kenneth Wolcott]

Hi;

what is the meaning of a note that looks like a rhombus in a
contrabass quartet arrangement?

I've attached a SCREENSHOT of small part of the Contrabass Quartet
arrangement of "On the Steppes of
Central Asia" (composed by Borodin) which I found on IMSLP.

I don't see anything mentioned in the Notation Reference regarding this.

Thanks,
Ken Wolcott
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