Bill and all,
I also think there were cars that had 'just the keystone' (i.e. without
the circle or the shadow) but I don't have a clue as to when those
were being done.
Thanks for the date range for the circle -vs- shadow. Yes, I
understand that a car with a circle keystone paint might have
lasted well beyond the introduction of the shadow ... it always
depends upon when the car was repainted.
However, there is another aspect to those date ranges ... if
your layout/ops scheme is set for "1953" then you should not
be running any shadow keystone cars. *G* But many of us
do not adhere to "era" as tightly as others ... and that's OK
if that is your thing.