We have customers calling with this 20-1624-4001 number usually about 3 times a month.   They are finding this number on the bottom of the door closer, usually on the back of the closer, so the only way to find it, is to take it off the door.   The number is telling us you have a Norton Door Closer 1600 (1600), new style (1624), with a size 4 (1624) .  The Norton Door Closer we send will work in place of it, but it will have an adjustable spring, with a hole spacing of 3/4″ x 9-1/16″.  Below are some of the different numbers we find and what should be purchased, just follow the link.
20-1621-4001
20-1623-4001  3/4″ x 9-1/16″ hole spacing
20-1624-4001
20-1603-4001  Not Available use this closer 3/4″ x 8-3/16″ hole spacing
20-1602-4001 Not Available use this closer 3/4″ x 8-3/16″ hole spacing