Mark Hammer (April 28, 1937 -February 15, 2007) appeared in several early Joe Frank programs. He taught drama at Catholic University in DC 1966-1989, appeared at DC's Arena Stage 70 times. 
He appeared on Broadway a few times: https://www.broadwayworld.com/people/MarkHammer/
and off-Broadway 9 times: http://www.iobdb.com/CreditableEntity/4089
as well as in a few movies: https://www.imdb.com/name/nm0358481
His hometown paper, the San Jose 'Mercury News', published an obit: https://www.mercurynews.com/2007/03/26/mark-hammer-veteran-actor
His brother is Philip Hammer, husband of San Jose's former mayor Susan Hammer; they're patrons of the Philip and Susan Hammer Theatre there.
He was married to Carolyn Swift, who appears in 2 of Joe's shows. I think she's Kathy in 'Rent-a-family', which would make him a convenient Arthur.
Mark Hammer has appeared in the following programs: