The Febi chain tensioners for the M117 engines (think 560SEL/SEC) are of demonstrably poor quality and do not last. It's not that different of a design from that of the M119.
As you noted, it's different than the factory unit and may not be correct in the line-up of the holes and passages and such, not to mention the quality of the interior spring.
Just sayin'. I wish you the best on this.