Look very carefully at the current exhibit at Orly’s site. The “6d” is much more sharp and darker than the 7s next to it. The “0” in the 47044 is not a zero, it is a schmutz “O”. The “E” in EF Lavender is sharper and darker than the “F” next to it.
My first typwriter lacked a number “one” and a number “zero”. You used a lower case ‘L’ for the number one and an uppercase “O” for the zero. I think they were all like that until they came out with automatic carriage return.
Gee, I guess they must have used a manual typewriter. You know - where each key is pressed by a finger. Have you ever used one?
The letters don’t all come out the same shade of black because the fingers cannot always apply the same pressure. And now over time the ink has faded.
When IBM came out with the Selectric and those odd little lettered balls in differing type, it was a huge improvement.
(Am I ever giving my age away!!)