Help me identify rain shower head control

I am trying to identify my control valve for my rain head. The only marking on it is W with either an 8 or a b on the upper left side of the W and then a 1. It is above the main control for the regular shower head, and is separate. It seems like it just controls the flow to the rain head no temp control. I will post a picture when i get one. Thank you very much for any help. Matt