Just one note to the topic. Professor Pavel Valíček, co author of the book Pěstování citrusů (Growing of citruses) issued in 1987 from which are the pictures, confirmed to me, that hybrid Unshiu x Poncirus was selected by professor Pospisil in seventies, the plant is still in greenhouse of university in Prague.
But I remember that Ilya posted at old forum article, that unshiu mandarins do not hybridize and cross pollination with poncirus is only used to obtain seeds to get nucellar seedlings, or mutants. So it is possible that plant mentioned above is chimera, or mutant.