Stampless cover of Zurich Canton "Five month without stamp again"

On March 1, 1843, Canton of Zurich issued the first stamp of European continent, however they have five month period starting in 1949 without stamps just after the change of national postal rate of Swiss Confederation because there are no stamps which suits for the national postal rate.
I recently got one cover of the period and show it here. This cover was cancelled at Zurich in the afternoon Jan. 24, 1850 and you can see a stamp "Franco" which means the postage to the destination is fully paid.  This cover was going to Eschbach, Baden  (South of Germany now) and the amount those days was  6 Kreuzer .
There are not so many international letters those days, so the postal clerk can not always tell the full postage rate to foreign countries. But as Baden is close to Zurich, the postal clerk knows the rate and that's why the sender of this letter paid fully and the cover has a red "Franco" stamp on it.
By the way, according to the Michel Guide of Stampless Cover, the amount the sender paid is written in red on the back of the letter, so I saw the back to find "6" in red.
I got this cover at 100CHF and this is one tenth-thirtyth of cover with stamps, though this cover is rare as it goes to foreign country with rare red Franco stamp on it. However I can enjoy with this cover very much as it has many points. As a matter of fact I can not understand the black cancellation on the back now. So I have to study it more.
I also want more common domestic cover for my collection and maybe it cost me much cheaper than 100CHF. I believe you can also enjoy Swiss Classics with small budget if you have knowledge.