Apparently, rotax pistons, manufactured by Elko KÖNIG, series 1800, 03967 diameter 79,5mm / 79,42mm 79,51mm were used in some of old-timers (I found references to: Aprilia, Puch, KTM, etc.)
There are 3 types: 1800a, 1800b, 1800c
Diameter: 79,5mm / 79,42mm /79,51mm
Height: ca. 63mm
Rings: 3
Rings spec: 1x Ring 1,5mm – 1x Ring 1,25mm – 1x oil ring 3,00mm
Material: Aluminum
Pin: 20mm
Top to pin center: ~34mm
Made in Austria – 011 – 79-6+
Weight: ~323 gramm
Where can I buy a set of the rings you have mentioned above? Thanks