My condolences on the loss of your husband.
While plate block collecting was popular in the mid 20th century, the practice fell into decline prior to the end of the century. Several years ago, my brother asked me for advice as to what to do with the stamps his wife had
inherited from her father, a retired postal carrier. I took a long look at what they had ... and told him their best choice was to use them as postage. Now, every Christmas, I receive a card from him plastered with enough old Xmas stamps to meet the current postage requirements. The only valuable plate block in the 1956-60 time frame would be the $5 black Alexander Hamilton mentioned in an earlier post. If you had some plate blocks from the mid to late 60s, you could check for tagging omitted errors ... but tagging on U.S. stamps did not arrive until around 1964, so this is likely not pertinent to what you have.