[ithacaartist]

If the upper moves back, try to rotate the take-down lever down to release the side plate from its position and move the top end forward and out of the frame. I'm thinking the entire upper might just slip out at this point.

If this can be accomplished, you'll be more likely to see what the problem is, plus have access all around it And at that point the rear toggle axle will be accessible. If it can be slid/gently tapped out from right to left, you're on the right track; even better if the breech block will slip back and out of the extension, using the other David's technique--at which point you'll also be able to gently apply a little force by hand to flex/rotate the joints. DJU's suggestion to soak it means, I believe, to immerse it entirely. This way, none of the kerosene will drip/dry off.

We can go farther explaining how to clean the joints before re-lubricating, but will wait until you report the previous steps are accomplished successfully.