Welcome to the Forum.
As noted,, your late father's Blackhawk is a first year,, of the NEW MODEL Ruger Blackhawk .357's. Made in 1973. It is not an "old Model" or a "3-screw" variation.
It has a little going for it as to value. It's a first year,, and a "pre-warning" variation of the New Model. In about 1977 Ruger started adding the barrel warning rollmarking that many here feel is horrible.
But real value of course is that it's a family owned gun. However,, for insurance purposes,, or estate purposes,, a lot depends upon condition, and completeness of the package. A gun of that vintage needs to be almost new to new condition to command the better values. And the original box, the shipper, the papers all add to value.
Since that model is still offered by Ruger,, (of course now with the warning rollmarking) it's value would be comparable to a current production one if in "as new" condition. You can add a little for the fact it's a 1st year, pre-warning of maybe 5%-10% if the condition is excellent.
You can look on Ruger's website,, looking at the current MSRP,, for an identical model,, and deduct about 25% to get a real world value if it's in excellent condition. Otherwise,, if the condition is less than excellent,, consider a lower monetary value accordingly.
^^^
