What can be done?

[LBPrepper]

[2Stroke]

Find a machinist who can make an adapter plate. It would have thru holes to secure the plate to the slide, and threaded holes to secure your new optic.

[Fatcat]

A Docter red dot sight (II, II Plus, III) will have the same hole spacing as a Burris Fastfire, or the VORTEX VIPER/VENOM.

[ar15barrels]

What does the maker of the slide have to say?

[LBPrepper]

I received the slide quite some time ago. I’m no longer in contact with the seller. I know where it was milled but not where the optic cut was done.

[ar15barrels]

I know where it was milled but not where the optic cut was done.

Ask the place that milled it then.