The Norton Door Closer 8301 is sometimes called the Omni Door Closer.  This door closer features a cast aluminum body, is a Grade 1 closer and has adjustable closing, latching, and back check speeds.  This closer is a non sized closer, meaning you adjust the spring tension to meet closing requirement for a ADA Barrier Free application all the way up to a 48” exterior door.  You cover might have 8301/8301BF 3301/3301BF, but the new version will cover the BF and non BF closers.  The difference between this closer and the 8501 is the size of the plastic cover.  The 8301 has a slim cover, and the 8501 has full cover.  The hole spacing on this door closer is 3/4” x 12”.