Additional knob/valve inside shower wall
First time posting in the forum - hope i get it right. I'm a plumbing novice and am not sure what the additional knob below my shower handle stem is. I've been trying to find out via the internet but it's difficult because i don't know what to search for. Most pictures of two-handle system (e.g. here http://www.diyadvice.com/diy/plumbin...m-compression/) do not show the lower knob or valve.
I'm assuming it is, or was (my construction is original 1971) common as these tear-drop escutcheons are something i've seen a lot and I assume were intended to disguise this additional part. However that is as far as my deduction has taken me.
Has anyone seen this part? What does it do? I thank you in advance.