This is the response i got from krebs.
Our AR15-style rear sight is attached to the receiver cover with screws. However, the rear of the recoil spring guide assembly must also be modified to stabilize the receiver cover.
If you have access to a gunsmith or a machinist, there are instructions for installing and modifying both parts that accompany the parts, or you could send your parts to us and have us do the work.
The sight itself is $65, and installation would be another $65, plus about $10 for shipping the parts back to you.
Please keep in mind that this is the rear sight WITHOUT the Picatinny rail section in front of it.
If you'd like to discuss this, or order the parts, feel free to call us at 847-487-7776.
Thank you for your interest in Krebs Custom.
Brian J. Conrad
Manager
Krebs Custom, Inc.