It was my fault. I thought, if there is no reference to an turboprop Yak-18 in the Yakovlev OKB history books, such a conversion could only be done by Chinese engineers (remember the original piston-engined Tu-4 and Be-6 later fitted with turboprop engines). But I was on the wrong track. The turboprop Yak-18 was a product of the OKB Yakovlev by itself! A good friend was able to name the source (Matej, my speculation was very close on it): A picture was in “Letectvi + Kosmonautika” # 15/1977 (pg 585), but unfortunately, my collection of this Czechoslovakian magazine starts with 1982. Can anybody from the Czech Republik or Slovakia help to post a scan of the picture?
In spite of my mistake, it’s very interesting, that the Yakovlev specialist(s) at Aerofax seems to be ignorant on this engine upgrade …