A Southern Pneumatics inline (male to female) reg, also known as a South Port or KC No Hot Shots reg.
I got a bunch of these No Hot Shots regs from Eric Scott and have been looking for compatible parts before I mess with them. Lower left image is the reg seat for the CCI revolution which I thought might be compatible. Little too small.
I believe these came out in 1993 or 1994 and were the first of several generations.
I also have black (earlier?) and grey/clear (later?).