Yes TC
it is the Letov Ljubljana, Slovenia, project Letov KBL-12 for a two-seat tandem trainer with a Turbomeca Palas 150 Kp engine. The designers were Ljubljana University of Technology students.
When yesterday looking for Walter's elusive challenge, I found this from a site that contained almost everything else except just what Walter had posted
.
TC has requested
Open House.
Edit: I did not find the Antonov in the web. Not saying that it is not, didn't try all tricks.
Edit 2 for Antonov:
An-20 (the second of that name) 1960. Competitor Yak-30, L-29.
3 views of books I. Sultanov, "History of the first domestic jet aircraft"