[Ger-Poland-Volhynia] Help Needed Identifying Birth Place in Polish Record

[Peter (chamdo4ever)]

Hi All,

I have the 1831 death record of a Peter Kramer from Płock. The record
mentions the place of his birth, and as best as I can see, it is
"Kostwa Posnajski."

Could anyone please tell me where Kostwa Posnajski might be? I've
searched high and low to no avail.

Images of the record itself and close crops of the word "Kostwa" and
"Posnajski" are available for viewing here:

http://imgur.com/a/3NQoI

Many thanks for any help,

Peter Schmidt