On the blackstrap under the grips below the hammer spring.

Indicates a Rev change in the part marked so. A revision. A change had been made in a part lot or string of parts. The change may be perminant now untill further revised. Frames prior to this may bare the Rev 2 stamp or none at all. I should add that the frame may be used for one particular model/variant of the P226 line and would be called out on a print. Rev 3 might be lacking or needing a certain feature machined into it to work properly as a certain model frame. A Rev change call out might require a total different part program to make, might simply require a block delete be added to a program. The call out might require different tooling or fixture. A number of things could be required.

You gave me the generic definition of a revision in relation to a firearm part, which I already knew.

What I am asking is does it have a particular meaning for the frame in question. For example, it might mean it had a stress relief cut made. Or it was re-inforced in a particular area at the request of an agency. Or that they were only marked so for a particular range of serial numbers or years or manufacturing location.

That type of thing.

I dont know where you would get that answer. Perhaps Engineering at Sig but it may be something they would not release. You can try by email to them.