I see that you have already received responses to your question.
To be specific, the abbreviation consists of the letters Shin-Alef. The Shin stands for the word "shem", name. The Alef stands for "Imo" (his mother) or "Imah" (her mother), as appropriate. Occasionally, there is a Vav at the beginning, representing the word "and".
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Fredel Fruhman
Brooklyn, New York, USA