I guess around 1980’s, because as I remember when my parents bought my the first mini bike this model was also in the bike-shop. Last year I also bought a PR6, but It’s very hard to get some pieces for such a bike.
I have a PDF with tecnical sheet from an advertisement of the PR6 if you want I can send to you…please contact me in any case.
If you have the intention to sell the bike, please let me Bid for it. Your bike is really in good condition, congratulations.