Interesting project smile

I know the add says it is safe, but first use a dowel
to insure the gun is totally empty.
MAYBE there are empty cases in the gun with firing pins
stuck in the spent primers.

Second, if empty, see if the top lever really moves the
needed locking parts when activated. You said the cross
bolt is not moving, can you activate it with a wooden push rod?

Third, cock the hammers and see if the firing pins can be moved to the rear.

Please keep us updated on progress

Mike