Decoding of pre-stamp covers

I started to collect stampless covers of Switzerland to understand  Switzerland classic philately much more. I've bought about 50 cover lots from Sqwitzerland and start studying them with the guide book of stampless covers published by Michel. Swiss Cantons of the age use different currencies and have each individual postal systems, you can enjoy domestic letters as if it is international letters. The Michel book teaches me "The paid amount is written at the back of the envelope while the unpaid amount is written at the front. "
When you see the cover below, you can see red  "6"  at the upper-right corner. I can understand this is the unpaid amount, however I can't tell the currency. The dealer said the currency is Kreutzer as it is sent in 1747 from the contents, but I need to study more. I have also difficulty in understanding where it comes from and the destination, thought the dealer said both of them are Zurich.

This cover is a folded letter and you can see "1747" at the top of the contents.