Looks like a Model 80DL or variant thereof from the late 40s or early 50s. I have a M80C made in about 1954 which is essentially the same rifle.
Open bolt and hold trigger to rear while pulling bolt straight to rear. The entire assembly will come out.
To replace make sure that bolt head is turned correctly (same orientation as when it came out) , hold trigger to rear to allow bolt to be inserted, and push it home.
Good little guns. The aperture sight is a nice touch.
My M80C was my first gun and I still have it. When my kids "needed" .22 rifles I went out and found them the same thing. I knew they were good and with a modest stock of spare parts they should be good for another 50 years.