I fixed it.
I took the bolt-stop block off the left side of the receiver. This revealed a large hole where I could peer into the action. I found that whoever assembled the rifle last somehow managed to push the bolt into the receiver, with the cocking lug in the bolt lug channel! The cocking lug them popped up inside the hole covered by the bolt-stop block. I used a long, thing screwdriver to puch the cocking lug backwards. When I had it back as far as it would go, I pushed in the safety and it twisted. Then it, the spring and the firing pin popped completely out of the bolt. The bolt then came out as normal.
Then, for the first time in my memory, I was able to look down the bore of this rifle. I must say, the bore is spotless. Shiny and crisp. There bolt assembly is likewise pristine. The spring looks absolutely brand new.
After a quick wipedown, I reassembled the bolt. I made sure before placing the bolt back in the rifle that the cocking lug was pointing straight down, and the handle straight up. The bolt slid in and now the rifle works perfectly.
Here are some more pictures for those interested....
Thanks for all the kind help!
BTW, this is a potentially dangerous rifle. You can fire the gun with the bolt handle straight up - with the bolt unlocked in the receiver.