I have not studied the base finish of PY Crown weights. And I have no PY Crown weights in my own collection.
However, my PY Caithness period weights are restricted to just eight examples, two of which have an indented and fire polished base.
In Colin Mahoney's book on PY weights, it is stated that about 80% of Caithness period weights have a flat polished base. Of the rest, "a few" (approx 18% according to the graphs) are fired polished and "once in a while" (approx 2%) "weights turn up with" a rough finish.
I agree with Sophie that only a uv test will determine the period of the example shown here.
[Edited to correct the percentage figure for "a few" - I originally said 8% ... should have been 18%]