Company wants to buy shares I didn’t know I had, for less than their value

I received a letter from a company I’ve never heard of offering to buy three shares of Brighthouse Financial from me at 2/3 of the current stock price. Apparently my mom who passed several years ago owned these somehow, through some insurance product maybe(?). I had no idea about these shares. The letter said all I needed to do was sign the papers and send back, and they would handle everything with Computershare which is I guess where the shares are parked. How would I as next of kin start to find out how to get these shares without going through this “middleman” company? Start with calling Computershare? What info would I need besides death certificates? It’s not a lot of money more the principle.