While the gun is apart, the top-lever spring may not have enough tension to spring the rotary lug over. When you depress the lug in the receiver, it will release the top lever and you can manually push it over.

I use the small brass wedge on a fishing line method of compressing the spring to allow reassembly. The hole drill and vise with a nail method didn't work so well for me. Here is what happens if it shifts a tiny bit:




Here is a pic of the brass wedge:



Best of luck, Rob.