A number of Anacondas were factory drilled and tapped...same with some King Cobras. I've no idea of the numbers done as such but it was a factory option. Ported barrels were an option as well...my 4" Anaconda is ported.
I don't have a feel for a fair market value...I would use as a bargaining chip that since it can't be verified the drilling and tapping is factory, it shouldn't be valued as such.