Yes, a mix of bolt styles is very typical. There are patterns of bolt styles and markings depending on the time of production and assembly plant.
In the course of cleaning and detailing the engine bay of my '65 Mustang (San Jose/February 16, 1965) I notice that there are two different style bolts with permanently affixed washers: One has a washer that looks like the classic "star" washer, and the other has a round washer with a serrated edge around the entire circumference of the washer's outer lip.
Ron,
In general the external toothed (aka star) washers were used in 1965
San Jose production. My May 65 Fastback build is 100 percent external tooth for the front body. I've owned the car for 18 years and replaced fender bolts (and other locations) with the same style washer and head marking. The three previous owners, the "original" and the two looking for a "quick turn" didn't do any restoration work so I am confident in the hardware accuracy. Over time, well meaning restorers and shops obtain almost anything available, hence the not so good "mix".
However, it's a different story on my Oct 65 builds, a Hardtop and a Fastback, they have a mix but with more disc than external tooth.
What was determined on the fenders for 65 and 66 was the first and last bolts were installed at one work station first from a bolt bucket on the left and a bolt bucket on the right. This was to facilitate adjustments. The car moved down the assembly line where the hood and grill sheet metal were installed and adjusted, from their own sets of bolt buckets, and then the fenders were adjusted. Further on down the line, the middle four fender bolts were installed from their own set of bolt buckets. That's the how you get a "mix", and it's a result of the process. The 16 bolts used on the spring shield (the proper name is "Bracket, upper A arm bumper) are a different length and were installed well before the fenders.
A March 66 San Jose Fastback I restored was almost 100 percent disc.
Remember, these are San Jose observations, Dearborn and Metuchen were different.
Jim