The SM668 was in production 1968-69, followed by model SM868 in 1969-74. Almost 1.800 units were produced.
The tractor Volvo T 600 (also sold as BM 350) was a base for the SM668. For SM868 the tractor T 800 was used. By the way, SM stands for "skogsmaskin", 'forest machine'.
Volvo BM SM668 ?
This looks like a Volvo BM SM868.

Volvo BM presented a new generation of forestry machines in the early 1970s, but not so many machines were made. Is this one of the few?

Volvo had decided to let the Finnish company Valmet take over the production of tractors, and after some time decided to let Valmet take over the production of forestry machines too.
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2007-04-10. www.konditori100.se. Text/pictures: Arne Granfoss ©. Prod: AG Informice