Here’s some info on the screws;
The gist, I think, is that the HL15 should come with 6x 10-32 1/4" screws for attaching rails regardless of if you order rails. I think there were some issues with the initial production re screws not being included, some screw holes not being tapped correctly on the chassis, and that the rail kits also include screws that are threaded differently and not needed. That should all be fixed now.
I have the 00053 rails and didn’t have any issues, so being uneventful, I couldn’t remember the details. I’m not exactly sure what the difference is between the 00053 and 00058 except the 00058 seems rated for taller chassis and perhaps therefore more weight. But there may be other physical differences.