It goes off of the height of the hub...the 36 is longer then the 37-39. That one sure looks like a 36 from here.
OJ this is a 36 hub, the neck measures 2.300", the only year with the "long skinny neck", as far as I know.
I could be wrong but I think 39 is the only year with the removable hub. Clark Edit...I guess I shoulda looked at Marty's pics and answer first. Didn't know the 36 had a remavable hub.