If your Door Closer is cast with the numbers 20 3002 4010 it is a Norton 8000 series or Yale 3000 series Door Closer.   We find this number on the back of the hydraulic cylinder, along with the numbers 149A and R668.  We also see T-40 on the back.  The door closer has a hole spacing of 3/4″ x 12″.  You also might find 3301/3501 and 8301/8501 on a sticker inside the cover of the door closer.  Basically, The Norton 8000 series is the 8301 and 8501, the difference is the cover.   The Norton 8301 has a slim cover, were the spindles stick out of the top and bottom of the cover.  The spindle is the shaft that the arm attaches to.   The Norton 8501 has a full cover which would cover one of the spindles.   For the Yale 3301 and Yale 3501, the 3301 has the slim cover and the 3501 has the full cover.   The Norton and Yale door closers are the same door closers, they come off of the same assembly line, only one is marked Norton and the other is marked Yale.   These door closer also might have the name Scoville or Eaton on them.   This is also true for the Norton 1600 and Yale 50 series.   Below are images of the Yale 3301 and Norton 8301, and the Yale 3501 and Norton 8501.

the above image is of the Norton 8301 or Yale 3301 door closer, notice how the spindle sticks out of the top and bottom of the cover

the above image is the Norton 8501 and Yale 3501 door closer. Notice how much larger the cover is.