Not originally flat but they are slightly cupped but not heavily (or double) cupped like you find on replacements. Think the zinc double cupped style is likely what your thinking since they are so commonly found and seen on older Fords.
Believe you will find that allot of the boots were ripped and damaged early in life so you will find allot of them have been replaced with the later deeply or double cupped washers on the inside and outside of the bracket. Often the plating is much nicer that other plated hardware of the same type on the same car.
Here is what I found
Examples of the slightly cupped washers retained on the whee side by a single nut. Have a fair collection of examples exactly like this from 67 San Jose cars. Would have to look at a lot more pictures and documents to see if and or when the double cupped washer was used on the assembly line. Just focused on 67 San Jose since that is the focus
Matching the service replacement kit at the top of the picture and two examples where the car got the boot repair kit. Notice that the kit included two nuts. A regular one and a sheet metal locking one
Picture of one of the kits showing the washers and other hardware that was sold into the 90's at least
Noticed that your car when you got it appears to have had the slightly cupped on rather than the one shown above
Opinions can and will vary
