I do know, that in the past 25 years, I've reworked more than a hundred locking bolts, And the only time I've EVER seen a top lever problem (as when a top lever has broken), was when the locking bolt was worn to the point that the gun wanted to open while it was fired. Many times when the top lever approaches that 6:00 position, or even further to the left, this begins to happen.
A new top lever replacement is not cheap. Preventitive mainenence is.
Jack (MIA) mentions I file the locking bolt for a perfect fit. I don't file. On the older Perazzis, I recut the lug on the bbl's to a slightly lesser angle (8 degrees). This gives me a nice square, guarenteed flat surface to work with.
When I weld the locking bolt, I do not use an extremely hard welding rod. I like to see as close to possible of equal hardness of the bolt and the bbl lugs, with the bolt just slightly harder. When refitting the locking bolt, it is ground on a surface grinder and a sine plate (this is an ajustable "angle making" magnetic plate). Given the now machined and ground surfaces, a very close to 100% contact is made by the two parts.
Some will suggest replacing the locking bolt, because the rebuilt bolt is imferior. I've yet to see evidence of this. What is fact is that rebuilding the bolt is much more less expensive that a bolt replacement.
A new locking bolt comes oversized. Material needs to be removed from the top, bottom, and both sides. Then the angle needs to be added. A new Perazzi bolt is over $100. The fitting of the new bolt could run from $75 to $125. A rebuilt bolt $125. This is because no extra fitting is needed.
It depends on how far back it does come. If the tip of the lever is over wood, it is too far right. If just at the wood/metal junction, it is enough for a good lockup.
When it is time for a new/rebuilt locking lug, your gunsmith will build up the lugs of your single barrel so the lever is in the same position for both barrels. If your smith says he/she will grind the O/U lugs down to the single's lugs, find another smith.
There's one thing to be careful about if you decide to mess with the lugs or the locking bolt. You need to be sure the bbl is seated properly into the joint roll pins.
Within the last couple of weeks, I had to fit 5 bbls and 3 receivers. None of the 5 bbls were original to either of the receivers. Though the bbls first appeared that they would have fit fine without further work, once I installed the forend iron to the bbl/rec assy, the pressure of the forend would pull the bbls deeper into the receiver. This was evidence that the bbls were not making contact with the pins.
One of the receivers was an older Comp-I O/U and the other two were of the MT-6 Style. It appeared that all 3 recievers did not have parallel inside walls. They all tapered tighter towards the bottom of the receivers and towards the standing breech. So as the bbl was pulled into the receiver from the pressure of the forend iron, they would get pinched and become too tight to open or close the gun. The only way to correct this was to work on the sides of the mono-block until the bbls freed up.
As the bbls freed, the bbls then also moved forward and made contact with the pins. At the same time, the top lever began to move to the left. On the MT-6's, once the bbls were seated properly, no further work was needed on 3 of the bbls, other than the resoldering of type 4 forend lugs and rebluing.
On the older Comp-I (MX-8 style), I had to rebuild the locking bolt, because the top lever eventually stopped at the 6:00 position on both bbls.
Now, if you're just trying this bbl out and not sure if you will commit to it, it shouldn't hurt to shoot it as it is.....just not too much. This is because the breech face seal is not as secure as it should be.
A little trick to make you think the top lever position is just fine, is to peen the bbl's lugs up so the top lever will sit to the right. This is a temporary measure. The metal just moves back rather quickly.