Hi John,
As far as I know, they NEVER were updated - when they started installing the aluminum V8's they stuck with the original subframe. If you want a strong boxed subframe you have to buy one from an aftermarket supplier. If I'm wrong on this, please correct me, but I've never seen a boxed subframe from the factory.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C