Hello Anubis,
I believe there were such things as 8/1 type engines de-rated to 6 hp at 650 rpm. If it is one of these it will not have the change over valve in the head, but
a square headed plug instead. In this case the alloy piston and 4 rings will be correct. Indian Listeroid spares are available to fit most CS types, and quality is OK. It's the
assembly methods that is their main problem. Hope you solve the mystery. Try decoding the year of manufacture. Any digits following the 61 on the plate are added to
1950 to give the year. Change over valves were discontinued somewhere in the '50's, when the 8/1 was produced, so you may have a de-rated one of these,
Regards, Combustor.