The strap is not adjustable and is 50" long.  This is from a small midwestern town auction so may have been an old military holder or possible used for purposes of the VFW, etc.  Can only speculate as to the origins. Needs some love and care. IT IS BEING SENT IN FOUND CONDITION. 

The strap is leather (approx  3/4-1") and the flag cup appears to be a plastic piping of some sort with a metal bolt through the bottom. The circular plate behind the pole cup is made of wood and is approx 6" -6.25". Inside diameter of the flag pole cup is approx 1.5".  THe length of the cup is 6-6.25".  

It is in used condition. It has wear and tear consistent with it's age.  The age is at least 20 years but beyond that, I am not sure.  A buckle could easily be attached by cutting the leather strap.