Pocket Watch Database
"1611" could possibly indicate the grade of the watch and not the serial number of your movement. Serial numbers are usually more than a few digits.
From the American Horology Archive:
Illinois Watch Co.
|Manufacturer Location:||Springfield, Illinois|
|Movement Serial Number:||1611|
|Estimated Production Year:||1872|
|Dial Feet Locations:||16.5, 36.5, 53.0|
|Case Screw Locations:||O-36.5, H-51.5|
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Data Verification Reports
rmwatchandclock 75 · September 10, 2020
Serial Number 1624 was reported as accurate.