The Redhawk was introduced in '79, and hit commercial production in '80. The SRH was developed in response to an issue with improper lubricant choice for the barrel installation, weakening/microfracturing the barrel tenon during installation, and was released in '87. By the time the SRH was released, the issue was identified and resolved, so the SRH really shouldn't have ever been released. So... If I were to hazard a range, you'd be looking at 501-30000 to 501-80000, signifying the years of '85 and '86.
I have a couple of Redhawks from 83, two from 84, and one from 85, which I've shot regularly with full "Ruger only" level loads for several years. I personally wouldn't hesitate to purchase any Redhawk from this range.