Stock sale

My husband was gifted 12 shares of stock as a bonus while working for Boise Cascade about 40 years ago.  We finally sold it last year so now I have a sale to report.  But I have no way of knowing what it was worth when it was given to us and he hasn't worked there for 45 years.  It was most recently managed by Computershare for ODP.  They took over in August of 2019.  How can I report this or where do I turn to resolve this?  I do have the 1099B.

Louise