Dr R.J.Ceresa in his work “The Postage Stamps of Russia, 1917-1923” (volume 5, section A, parts 1-5, p. 126) pictures a cover from Korets (27/7) to Novograd-Volynsk (28/7) with franking identical to yours. Could it be your cover?
As A.Epstein pointed out, we are dealing with an area under national Ukrainian administration (the Hetmanate) at this point in time. The intercity letter rate was 25k, so your cover appears to be properly franked.
In my collection I have a large-sized cover posted on July 23rd, 1918 from Korets to Novograd-Volynsk (24/7). It is franked at 32k (!!) with 4x5k fiscal stamps + 4x3k imperf Imperial Arms. The rate does not make sense to me, but….