So as builders, we all have options and choices in hardware. After 42 years in the heavy jet business, I've seen as many fastener types. Some are great, some are horrible. Building my RV-4, I used all reduced head titanium phillips head screws...the next owner (if there ever is one) will get a goody bag of spares, otherwise, he will post on here what the &^%$ kind are these. In day job we used a lot of Tri-wing and Quad-wing (offset Phillips), as well as Hi-torque Coin slots, and weird one off type fasteners not well known to the free world. Certain individuals in the industry , have easy access to procure such unique fasteners, and some are more durable than a standard Phillips head..as long as you have the correct bits. The Airbus products carry a lot of the Quad-wing offset Phillips..just saying, maybe the builder was an Airbus guy.