Just a theory, but I suppose that the over-stamped number 69818 could have been discovered to have already been used on another gun in the production process. After all, they are only 51 numbers apart. Quite likely that they were built during the same week. Two mixed up work orders, perhaps, and a little steel erasing to correct the error. Perhaps the error occurred with one being started in Syracuse, and the other started during the transition to Ithaca. We'll likely never know. Maybe an employee even got to purchase the over-stamped gun at a discount.
The F on the barrels is no surprise. There are many Lefevers that have barrels which are stamped a grade higher than the gun. But I have never seen one with a barrel stamp of a lower grade. It seems they would occasionally use higher grade parts to complete an order, but never do the same using lower grade parts. This may also explain some of the DS grades that are seen with dolls head barrels, cocking indicators, and LACo. monogram buttplates. You might get more than you paid for... but never less.
Some may recall that several years ago, an Optimus and a G grade both appeared for sale on Gunbroker at the same time, and both had identical serial numbers. What are the odds of it happening in the first place, and what would the odds be of both showing up on the same internet auction site, at the same time 100 years later? This anomaly caused some to question the authenticity of the Optimus, but experts who examined it agree that it was legitimate. The same guy wisely bought both guns. Hopefully, they will never be parted because they make for a most interesting Lefever mystery.