Do you know this hybrid ?
According to exotickerostliny :
http://www.exotickerostliny.cz/en/katalog-rostlin-othermenu-95/citrusy-othermenu-90/78-hybridni-mandarina-a-hybridni-citrusy.html
"'Atalantia x Upavanskij' is a cross of the genus Atalantia (close relative of the Citrus genus) and the orange variety 'Upavanskij'. It was brought to Central Europe by professor Mamporii from the fromer USSR. It is a very interesting plant with shrub-like habit. Young branches and shoots led to grow horizontally are extremely vigorous. 'Atalantia x Upavanskij' has numerous flowers that grow mainly in clusters of 4-5 and appear next to almost every leaf axil. Lots of flowers make this variety very decorative. Its fruit resembles tangerine fruit: it is 5cm long in diameter and has dark orange rind with almost no albedo. The pulp is orange as well and has pleasant, sweet, slightly sour flavor."

I have one, and I had ONE fruit this winter.




It's a very good mandarine with a taste between sweet orange and mediterranean mandarine (avana type).[url][/url]

Eisenhut nurseries in Swizerland says A. citroïdes.
http://www.eisenhut.ch/pflanzenmenue-f/citrus

The image of this hybrids foliage reminded me a lot of this one of mine, an off type Rangpur seedling
link

Are you sure that is really an Atlantia hybrid ?

I'm absolutly not sure. It came from former USSR, throw eastern countries by non official means so the origine of this cultivar is not warranty. We have a false sudachi and a false yuko from USSR here in Europe.
However, it´s a very good mandarine.