Actually, if those wheels are the same base wheel as the W23 Class II 16 Slot wheels on my '70 Monaco Brougham, the center area is the argent silver, the slotted area is chrome, with the outer rim (covered by the trim ring, normally) in black (with the rear of the wheel being black, too). You can find the 1970-71 Chrysler Parts Manual at
www.mymopar.com, click on the "Tools and Reference" link on the lh, then on the Parts Manual link on that page. When you get the parts book downloaded, you can navigate through it to the wheel section (page 22-4), where you should find everything you'll need to know what was what. Looks like a different center cap applique and also a special trim ring, as the sales brouchure illustrates.
The wheels should be 15x6 in size. Chrome lug nuts are "solid". It was not unusual for the factory lug nuts to have broken chrome on the edges of the flats, due to wrench contact. They'd actually break and peel back, with time. Seems like there were some aftermarket lug nuts which were "reasonable facsimile", but a little taller? Should be all rh-thread on your '71.
From looking at the part numbers and such, it might be that the Plymouth SFGT wheels were all black, as the trim piece will cover the wheel, as something like a multi-piece wheel cover, so no need for any chromed area (as the normal trim ring wheels had).
The BIG question might be . . . is it the orig 440 4bbl?
Those were neat cars and low production cars. Certainly enough there to justify putting it back "right"! Good luck on your project!
CBODY67