I don't have a definitive answer and can just refer to my experience. I have seen this often in parish registers when referring to how something is said, spoken or generally known e.g. like house names. In a wider sense it may be also used to refer to something which is known by reference. In your example we see a reference to another (legal) document. Such references often had to be copied to the register word by word so it might being used as our quotation marks there.
Some examples for the usage of `/: ... :/` in parish registers in the Diocese of Gurk, Austria:
* < (first entry, referring to a house name)
* < (fifth entry, referring to "the one above")
* < (first entry, referring to the number of documents)
* < (first entry, used for house name and for additional information about the birth date)