Regarding deeper "1" digits than the others. . . . .
If you look closely, numbers in post 8 that have longer lines to make it are shallowere than those with shorter lines. That's cause there's more resistance to the stamp with them. Short lines go deeper than longer ones. The dies for each were all struck with about the same force from the hammer.
If you stamped a "period", or "." in the serial number area striking the die with the same force, it would be the deepest one.
This is more visible with softer metals.