Hi, Been a few years sense I worked on a Symmons. They had a small lever at the bottom of the face plate that turned a diveter. They wqere always getting lost or broke. The repair we made was to change the spout with one with a diviter, that worked for us. To see if this will work for you turn on the water and try and stop it with your hand the water should come out the shower.
Good Luck Woodbutcher

I wound up grabbing a new Temptrol at H.D. and grabbed everything I needed off of it, including the seats. As I removed the outer (cold) seat I found the steel washer from the original cartridge wedged inside it. I replaced the seats, cartridge, cap, and trimmed it out, works as new and cheaper than buying individual parts.