DARPA developed the XM-3 for the USMC around 2005. The rifles were built by Iron Brigade Armory. Here’s a link for additional information. Enjoy!
https://www.snipershide.com/shooting/threads/xm-3-on-cmp-what%E2%80%99s-the-story-behind-these.7114226/
Are you looking at the auction / rifle he referenced?
The auction he posted looks like a genuine XM-3, and there’s no M24 iron sight mount on the side.