I may be the bearer of bad news here
The little circular plug with the two holes in it is not a lock. It is merely a threaded plug used to access/retain the firing pin block parts in the slide, used on early production models. The holes are presumably to insert a special spanner wrench into. I suspect they had too many problems with parts flying out of the slide while shooting, as mine did. Not only did the plug come out and get lost (presumably mixed up with the brass on the range floor), but so did the firing pin block plunger, and probably some other parts (e.g., springs) as well. My replacement gun does not have this part.
If it gives you trouble -- as I suspect it will -- Sigarms does at least have good customer service.