I've personally never heard of an SWS brand timing chain, but that doesn't mean much. However IWIS brand IS OEM Mercedes so you're good there. I've never heard of checking for chain stretch by removing the tensioner first. I'd have to say that's not the right way. I've always been instructed to do it pretty much just the way you said you did it, however with the tensioner in place.

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Dedicated to the preservation of antique Mercedes Benz's, one rusty ol' bucket of bolts at a time!