Sorry, folks, but that Colt belongs to an era where the numbers on the frame and the crane WERE the serial number, not an assembly number or (to be more exact, the serial number was used as the assembly number).
If it is the Colt .32 New Pocket, serial numbers started with 1 in 1893, and number 18 would have been made the same year. That gun should be marked "COLT D.A. .32" on the barrel and "COLT'S NEW POCKET" in a circle on the left side of the frame under the latch. It should have the old style frame with a straight part above the trigger guard rather than the curved cuts used later.