What constitutes a "First Issue" Cobra? I suspect that term came from the "Blue Book." The term "Issue" is apparently a creation of the "Blue Book," and is next to worthless for describing a particular Cobra. The term is not accepted or used in the Colt collector community.
The Cobra has a much shorter history than the Detective Special because it did not exist until Post-War. The Cobra was introduced in 1950 as an alloy-frame version of the Post-War Detective Special, already in its Third Generation by that time. These early Cobras had the “Dual Tone” finish and plastic stocks. I would call these guns "First Generation." The plastic stocks were changed to wood in the mid-1950s, and the “Dual Tone” finish was dropped. I do not consider the stock material or finish change as another generation, but some might. In 1966, the butt frame was shortened, which is Second Generation. In 1973, a year after the Detective Special got the shrouded ejector rod, the shrouded barrel was introduced on the Cobra, which I count as Third Generation. The Cobra last appeared in a Colt catalog in 1978, and was absent from the 1979 catalog.
Therefore, in my opinion, a "First Generation" ("Issue" if you insist) Cobra has to have the long butt frame and un-shrouded ejector rod, NOT just an un-shrouded ejector rod. With the plastic stocks and "Dual Tone" phasing out in the Mid-1950s, there are important variations within the First Generation Cobras that should be identified. Obviously, a 1950 Cobra is worth a lot more than a 1965 Cobra, even though both are "First Generation."
In order to estimate market value, the year of the Cobra in question is needed.