What type of fitting connection is this?
It appears to be a 1/2" threaded stop plug with an Allen wrench shaped hole in it . It also looks like the plumbing fitting itself may be 3/4" itself , with a 3/4" to 1/2" reducer in it.

The fitting, the way you hold it in the picture does give us a shot of the other side of it... Would need to see that too.