MC-5D is made from a regular SKS and uses a modified regular trigger group. Larger capacity mags can have issues staying in place. NR, Sporter, and M's use a short gas system and are made new (not from regular SKSs), and use the AK style mag latch. The D and SKS-30 have a more military look and are 20" barrels along with the AK mag latch. The variance in designation is largely due to time of manufacture/import as well as who the importer was.