So if I have got this right, you can not see the actual black number through the print, as shown by your not seeing it when you have a black easel.
but when light is reflected back by the white easel, You can see the number.
So the number would be appearing whiter? since the reflected back light would make the print darker everywhere except the opaque number, where no light would pass through to fog the print.
Is that right ?
Entirely correct! You can see the marking as a slightly lighter tell-tale sign in an otherwise medium gray print.