This is from a rather well-known batch of covers from Korets to Novograd-Volynsk under the authority of Ukrainian Hetmanat. They are philatelic almost without doubt but this could be called forth because of a probable shortage of normal postage stamps at Korets at that particular time.
These are stamps of the Russian ‘white’ North Army in the Northern Russia (autumn 1918-spring 1920).
They are considered mostly bogus but in my opinion, they could be prepared for issue but not released because of evacuation of Allied troops from this area and defeat of the North Army by the Reds. The cancels on them are fantasy.