Head is similar to a round, slotted head, except where the arrows point is like it was cut out. It is on an old double cylinder deadbolt. Wondering if I’m going to have to drill it out.
Hard to tell from the picture, but based on the description, that sounds like a one-way screw. See this page for some more information. Basically, the design is to make it easy to install with a regular screwdriver, but hard to remove, which is what you want on a lock. To remove it, you may need to use a screw extractor - basically you drill a hole into the screw, grab it with an extractor, and then unscrew the combined extractor + screw.
These type of security screws are designed to be difficult to remove. While a screw extractor will work it may be easier for someone with limited experience to drill the heads of the screws off using bits of increasing size. Use a center-punch to dimple the screw-head so the drill will stay centered. Start with a small bit (1/8"inch), gradually increase in size until you are using a bit the same size as the head (5/16"). The heads are tapered so do not drill too deep or you will remove the countersink in the lock. Drill just deep enough to go through the exposed head. Then remove the lock. I would grab the screws with vise grips and twist the screws out of the door.