Unless I'm mistaken, the s/n should have 9 digits; the first one indicates the country of manufacture (1 = Germany), and is usually represented by an 'X' in the IPL.
You can find an IPL here, btw:
https://www.arboristsite.com/threads/stihl-illustrated-parts-lists-ipl.317358/post-6461559 ).
The s/n you mentioned has 10 digits, are you sure it's correct? Or is it a part number, maybe? The s/n should be here:
View attachment 1214610
For some models, the 'Magnum' label actually makes a difference, displacement wise, but not in the case of the 066:
Quoted from
https://fireandsaw.com/stihl-066-chainsaw/