My son has been cleaning and fiddling around with his Mosin and discovered something which he thinks might be a kind of safety.
With the bolt cocked and in the fire position, he could pull on the mushroom-shaped knob on the back of the bolt and then rotate it to the left side to catch on a part of the receiver which seems to be relieved to hold that part of the bolt.
Update: As I was typing this I was also doing some googling on Mosin parts and found the YouTube video below by Hickok45. It is a safety as shown at about 9:20 in the clip.
With the bolt cocked and in the fire position, he could pull on the mushroom-shaped knob on the back of the bolt and then rotate it to the left side to catch on a part of the receiver which seems to be relieved to hold that part of the bolt.
Update: As I was typing this I was also doing some googling on Mosin parts and found the YouTube video below by Hickok45. It is a safety as shown at about 9:20 in the clip.