Common sense comes into play here regarding clarity of the common usage of 
'c.' for circa it boils down to placement in most circumstances. If one sees 
a 'c.' before as in:

c. 15th
(and most likely that above would be followed by the word - century)

it would certainly be clear that it was meant to be circa. Who would take 
the 'c.' in that example for anything other than 'circa.'

If after, as in:

15th c.,
it would certainly refer to century in this example.

Why would one ever use that last example to mean '15th circa' which simply 
doesn't make sense.

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