Another rarity is this cover.

20øre cover from Copenhagen to Riga in Russia (at the time)
sent to Gustav H. Wolff who was a German sculptor who visited Russia many times on study trips to learn the craft.
the cancel used is experimental clock cancel used in a very short period in 1892, consisting of 2 clocks with mechanical "clock arms" that could be adjusted to show the hour and minutes as well of course as the date to the side.
the reason it never made it past experimental state was the ink that hardened caused the movable arms to get stuck and would thus require endless cleaning and maintenance.
naturally as an experimental cancel not many covers exist, and they are quite rare, and many forged exist.
but don't go looking for them without this warning:
rarely they're seen on stamps with fractions of the cancel (most likely legit) on a pair with more or less the entire cancel (without a doubt a forgery)
few entires have been seen, sent locally in copenhagen (most likely forgeries)
so be careful what you pay for such items.
this cover is real, it's one of few covers that has made the trip in the world.
sent from copenhagen 26th of october 1892 (gregorian calender) arrived in Riga on the 17th of october 1892 (Julian calender) 13 days behind the gregorian calender making the trip to russia in 4 days
and as such it's not a forgery.
on the back however is the stamp of the sender O.V.Riise who was the first chairman of the first philatelic club in denmark KPK.
O.V.Riise worked as a customs inspector in the danish postal service, and as such it would be likely to presume that his interest in stamps and contacts in the postal service would mean that he had interest in "making" covers for him self or collectors. which speaks against the legitimacy of the cover.
however, stamp collectors also send covers to people.. the affiliation between O.V.Riise and Gustav Heinrich Wolff is not known, neither is it know if the latter was a stamp collector. which, besides the fact that the cover made the trip, speaks for the legitimacy.
in either case, a beautiful and rare cover which is a priced object in my colleciton