Hi Jackie - My thoughts were similar regarding the top rim, certainly not one of the early buttons tops, but certainly much smaller than some of the later wider rims. Also as Sue mentioned in an earlier thread the shape is certainly more ovoid and doesn't have the wider shoulders as seen on some of the later examples.
So on balance my personal guess would be not very early, but reasonably early, probably around 72-74. Certainly after this point the form started to change.
Interested to see what others think on the date.