Doug's answer is good, I'll add to that by suggesting DNA tests. There is an Elkins Y-DNA project at FTDNA, https://www.familytreedna.com/groups/elkinsdna/about , joining it and/or contacting the administrators could help you find answers. You could also do an autosomal DNA test at either AncestryDNA (Ancestry.com) or FTDNA, or 23andMe. I prefer FTDNA, but doing the test with AncestryDNA gives you the option of transferring their results over to FTDNA allowing you access to both databases.
Since you are trying to identify or verify Leonard's father the Y-DNA test would most likely be the most immediate help for the specific question. You could add the autosomal at a later point in time.