There were some 5 slot argent wheels that are 1970 only. They take the same cap as the 68-69 GT wheels. They came in 6 & 7 inch widths. If you can see them on my signature picture thats what they look like. I need one for a spare if someone has one cheep.
You could get argent styled steel wheels from 68-70. I know that you could get them on Torinos/Fairlanes, but I suppose you could get them on just about any Ford car. Without my books, I can't confirm if 70 was a larger wheel though.
Oh and the wheel wasn't specific to Mach I's if you got them on a Mustang.
Think you might be confusing two different wheels First the 16 ( think it was 16) holed wheels what were available in 68-69 and the 5 hole wheels that were available in 70-71 (dependign on car model
The 16 hole wheels were available in three different styles over that two year period - Chrome, argent and with trim rings for each hole The 5 hole (similar to Chevy rally wheels) was available only in the dark (charcoal) argent.