I have a data table like this (Table1)

Sequence Date Time ExternalSymbol FirmId
1 20060929 9:30:18 RUPL06C95.00 BOX549
2 20060929 9:30:18 CYQA07C27.50 BOX017
3 20060929 9:30:22 NQ A07C12.50 BOX917
3 20060929 9:30:22 NQ A07C12.50 BOX017
5 20060929 9:30:22 RUPV06P90.00 BOX549
5 20060929 9:30:22 RUPV06P90.00 BOX017
7 20060929 9:30:24 RUPM07P70.00 BOX017

I am looking for a formula for COL A of Table 2 below that will look at
Data, Time, and External Symbol, compare to Table 1 above, and if all 3 match
for a given row, return the sequence number from Table 1 or return "Not Found"

Table2
Sequence# Date TimeConv FirmId ExternalSymbol
1 20060929 9:30:18 BOX017 RUPL06C95.00
Not Found 20060929 11:08:41 BOX017 HD N07P32.50
Not Found 20060929 11:16:58 BOX017 QAAD07C85.00
Not Found 20060929 11:35:39 BOX017 CCQD07C37.50
Not Found 20060929 11:44:15 BOX017 AAOD07C40.00
Not Found 20060929 12:26:32 BOX017 KDUV06P160.00
Not Found 20060929 12:26:35 BOX017 MMMP07P65.00

Thank you in advance.