Officially the same rules apply to side match guns as main match guns, ie they must be of a pre-1899 design. The first Marlins with round bolts were the model 336's manufactured in 1948 so they wouldn't qualify. The confusing thing is the model 1895. All original design model 1895 Marlins have the square/flat bolts, however Marlin introduced their "New Model 1895" rifles in 1972. These round bolt 1895's are built on essentially the same action as the model 336. This doesn't necessarily mean that you wouldn't be allowed to use a round bolt Marlin at a local match, it just means that they don't meet the NCOWS criteria for an approved firearm.