[MRCA] SSS vs SOS?

[W2HX]

I came across this item on ebay
https://www.ebay.com/itm/265903930626

On the right, it seems there is a selector for "SOS", "SSS" or "KEY" I am making an assumption that KEY allows a person to transmit a message of their choice using a KEY. The SOS I assume sends an automated SOS signal. What is SSS? I know earlier in morse CQD was a distress signal. What is SSS?

73 Eugene W2HX
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