Sure that isn't just the 3500HD's?
100% positive.
well it cant be 8600 gvw as that is the 8 lug 3/4 ton gvw, this was a single wheel reg cab k3500
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That's kind of like saying there can't be 2500's that aren't 8600lb gvwr. The badge is just a badge and doesn't correspond directly to a tow rating or a GVWR. The actual rating is whatever GM decided matched the frame and drive train. I would think you would be the first to know GM did quite a few odd things in the 88-90 trucks.
In any case, if what you say is true, yes I need to amend my statement. All 3500 badged trucks did not receive BW cases, a majority did. Beyond that, without baring all trucks of the transition years I can't dial it tighter.
I would still venture to guess the GVWR numbers would shed light on the situation. There's no reason for GM to use BW sourced cases in bigger trucks if they could have just kept using NP241's.