Try relieving the top of the spindle a little till the top lever pin bears better on the top lever.
failing that to remove play a temporary solution may be to remove the spindle, reinstall the top lever pin to protect the threads - upset some metal on the square axle which will tighten up the fit a very small amount of shim stock may achieve the same - we are probably talking of play of 1 or 2 thou its going to be tiny - but the top lever is exaggerating the play in the spindle so its going to feel worse than it really is.
The issue wouldn't worry me as long as i was confident that the under bolt is properly engaged. I would just inspect the mechanism for your own piece of mind to confirm ( as i suspect ) that what you are feeling is a manifestation of wear and tear and not because its going to fall to bits.
Its an interesting point for discussion and one i would like to see some experimentation on. As i understand it the forces actually trying to wrench the gun open are quite minimal- despite years of development of hidden bites and extra secure locking mechanisms I've not read any texts of guns wrenching themselves open - many westley richards guns were made solely with a dolls head extension and a hidden bite with no under bolt.
I Have one experience of bringing the a gun so quickly into my shoulder and pulling the trigger when pigeon shooting that i am almost certain that i fired before the under bolt had engaged - this was with an over under gun and on firing i felt a force trying to push my leading arm downwards that i would describe in weight similar to holding a heavy house brick out at arms length. It was an interesting experience - at the time it shook me a bit but i remember the top lever thudding across distinctly after i had pulled the trigger.
