Thanks folks, very helpful. Just checked and my version is also 2.5.2.20143. Also checked and the Gunsight is present in Tutorials, but not in Instant Action, just as you suggest.
I'm coming to realize that DCS is a patchwork of not necessarily coordinated pieces rather than a coordinated whole. Explains a lot and is a good thing to know.
Hmm, perhaps modifiable in the mission files, something to explore.