Welcome Kim;
The #9 machine has the smooth plate, while the D-9 has the one with the finger hole cutout. The rolled down one actually protrudes slightly beyond the bed & has a rolled edge which is grasped by the fingers.
As a general rule the two plates are of a different width where they fit the machine bed, thus are not interchangeable. That said I do have a very early D-9 which uses a plate which is sized the same, thus interchangeable with, the #9 plate. It does have the finger hole rather than the smooth top with rolled edge.
I would have to double check to be sure but as I recall the #8 plate would not be interchangeable.
With the 1890 patent date your machine would have been built between 1890 & 1895, at which time the D-9 replaced it. I have a #9 which I acquired from the Granddaughter of the original owner. She knows her Grandmother married in a 1892 & that she acquired it after she was married, but doesn't know how long afterward. Its SN is 229,218.
All #9 machines have a SN of 6 digits or less, while all D-9's have a 7 digit number which begins with either a 2 or 3.
The #9 is a great machine & they sew very well indeed. Do feel free to ask any questions which may arise. Sharing information is our sole purpose in being here.
Miller/TN