I'm fighting some air infiltration from under my exterior doors. I've noticed that this sill cap / threshold is unlike any I can find in stores, as it has an indented channel in the middle. Is something meant to go in there to better seal to the door bottom?
You're missing parts there. No threshold has an open channel like that. At a minimum there would be a rubber strip across it. Most have a solid cap over that area with captive adjustment screws in it.
You can't really just replace a modern threshold with generic parts. It's part of a proprietary door system and it's fairly locked in place. Find the manufacturer and order the correct parts.
