Older 235/320/360 hydraulic cylinders simply used a washer to hold against the snap ring. Several of these older cylinders has disassembled themselves over time. The washer starts to bow and forms a bit of a ramp that encourages the snap ring to retract and disengage.
Later hydraulic cylinders replaced the washer with a machined part that captured the snap ring such that it cannot disengage. In order to remove the snap ring the new end cap is pushed into the cylinder beyond the snap ring groove. Once the snap ring is in place, the end plug and cap are pulled back out against the snap ring.
End caps are available for $15 each which includes the inverted style snap ring.