Melinda, The M91 Dragoon was intended for mounted infantry and the M91 Cossack for cavalry. At 48 ¾” they are 2 ½” shorter than the M91's 51 ¼”. The primary difference in the two is the Dragoon was sighted with the bayonet attached and the Cossack was not issued with a bayonet. More details here : http://john1911.com/mosin-nagant-91-30-ex-dragoon/

The word Dragoon definition- a member of a European military unit formerly composed of heavily armed mounted troops.
So its just designating what model these were ax different types of weapons configurations were made for different types of soldiers.