Will a Holosun 507 fit in the 226 Legion optics ready from the factory? Or do you need a plate to make it work.

There are many aspects of "it depends".

If you want a factory option without sending the slide off to be milled then you need a Legion RXP model that includes the additional holes drilled and tapped by Sig for the RMR mounting pattern.

The Sig RXP mounting pattern is specifically designed for the Romeo 1 Pro / Leupold Deltapoint Pro. That pattern is larger front-to-back than the body of an RMR or Holosun optic. An optic should be mounted using either a tight interference fit with the slide or the slide having milled recoil bosses to absorb the load. You don't want the screws doing all the work with front-to-back sheer forces.

Sig does not provide recoil bosses and an RMR or Holosun in an RXP mounting pattern, regardless of the proper RMR holes, does not provide an interference fit. Based on that, yes, you should buy something like the CHPWS filler plate as long as you can find the slide with the proper holes in the first place.

ah3, Thanks for the info. I thought it would be easy but I guess not. Will do some more research.