It doesn’t appear that this attorney was part of the monied Briggs family.
Allen (not Allan) Briggs grew up in Mount Clemens, Michigan, and passed the Michigan Bar in April 1957. He then became an assistant prosecutor in Macomb County in the mid 1960’s until 1966. He then moved to the upper peninsula (Ontonagon County) in 1971, where he became the Upper Peninsula’s first bachelor adoptive parent–of two boys, ages 11 and 13.
This article says that at the time (1971) he was the Ontonagon prosecuting attorney.
Six years later, his adoptive sons then ages 17 and 19, there were allegations that Briggs sexually assaulted a 15-year-old boy in Northern Michigan, but he faced no consequences and moved to an even friendlier jurisdiction.
He died in 2009 in Thailand. I think this obituary fills in a few gaps, even though there were no “file photos” to print:
https://www.pattayamail.com/838/obituary.shtml
Thanks again to Reader #2 for this information.