Your chassis is a "standard Lippert build" and was probably "spec'd out by Keystone" for some of the optional accessories that they include in some of their trailer lines, but not on your trailer line. Possibily holes used on an Cougar trailer or a Bullet trailer, but not on "your trailer" but all three use the same Lippert chassis.
Some of those accessories are a grill mount, a slide out bike rack/storage rack, a fold down bike rack/storage rack or maybe a spare tire rack (mounted on one of the chassis rails and both rails were "drilled for the mount" so Keystone could use the chassis on more than one "rear cap type" (windows on one side of the rear cap, they can move the spare tire to the other side so it's not blocking the window)...
Here's a link to the Lippert storage rack manuals:
https://lci-support-doc.s3.amazonaws...cd-0002319.pdf
And a photo of how one type of "OEM cargo rack" was mounted using similar holes on an older production of a Lippert Frame: