SL. NO. , so guessing it might stand for serial number ??

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Note added at 1 hr 25 mins (2005-07-31 06:55:10 GMT)
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These are some definitions you find for SL on the web:

The complexity class SL; see SL (complexity)
Sierra Leone, ISO country code
sine loco, used in bibliographies to indicate that the place of publication of a document is unknown
special linear group, an type of algebra
Storstockholms Lokaltrafik, public transport operator in Stockholm

Also:

SL = scrollbar list (linux); also,
SL = scrolled lines (linux); also,
SL = service level (pstn); also,
SL = service logic (pstn); also,
SL = Sierra Leone (inet); also,
SL = signal label (sonet); also,
SL = signaling link (ss7); also,
SL = slanted (fnt); also,
SL = slave (pc); also,
SL = slow (ic); also,
SL = space launch (sc); also,
SL = space limit (qa); also,
SL = Spacelab (sc); also,
SL = standard label (gpc); also,
SL = stored logic (pstn); also,
SL = synchronous layer (mpeg4)

But with NO. after SL., it really sounds to me like \"Serial Number\" :)

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Note added at 4 hrs 41 mins (2005-07-31 10:11:16 GMT)
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Check the link Ljiljana provided below, SL is used in the table to refer to \"serial (NO.)\":

http://dipp.nic.in/ut_hp.htm