Recently, my father passed away. My mother is his sole heir (as is proper) and I am her only child, helping her navigate through to her new situation. All of us live (lived) in the United States our entire lives.
Most of the finances are well-understood: There is a clear, simple uncontested will leaving everything to my mother, and most assets were jointly owned anyway. Life insurance policies, social security papers, pensions, and a reasonable middle-class managed financial account seem in good order. The size of the estate is small enough for federal estate tax exemptions.
There is one exception: In the process of going through his effects, we have found physical stock certificates acquired generally in the early 2000s. They happen to be for various US-based telecommunications companies, and even at a glance I can tell that this can become potentially complex due to splits, acquisitions, renaming, etc. (As the roughest of estimates, it seems to be worth about 10% of the separate managed account, so, by our standards a substantial sum.)
We've also found one letter (to Computershare) directing the sale of a small portion of these stocks in the mid 2010s, so we know my father was at least slightly active with these stocks.
Here is the problem: That one letter directing sale, and those stock certificates are currently all we have to go on; we have no guarantee we have found or ever will find everything; and I know less than nothing about the details and mechanics of owning or trading stock as certificates.
How is it possible to fully, efficiently, and frugally (in that order-- a full accounting is best even if it takes longer or costs a bit extra) understand his stock holdings? Bear in mind that we're uncertain if he used Computershares exclusively, or if he bought other stocks (with certificates or online) or if he had one or more brokerage accounts we are not aware of.
If the answer is, "Get a lawyer," or "Hire outside help," please be so kind as to include a description, i.e., "You need a lawyer specializing in X, Y, and Z," or, "The type of firm you are looking for is often referred to as..."