I didn't want to answer a challenge to my own statement but...
There's no confusion with the VIN (or Vehicle Identification Number "numbers").
I personally own a VQ Caprice with it's original running gear and the engine number starts with CVUxxxxxx, plus I have listed many examples over the years of "SVU" and "CVU" prefixes from other post-VQ V8 Statesman and Caprice vehicles respectively.
Statesmans have, since the beginning of time, had unique engine number prefixes to distinguish their engines from the lessor models.
HQ to early HZ Statesmans were prefixed with an "11Qxxxxx", later HZ Stato (after roughly 10/78) had an "11Zxxxxx" prefix, WB Series I had an "11Wxxxxx" prefix and WB Series II Statesman engines had a "12Wxxxxx" prefix.
The practice then wasn't used for almost a decade after the WB model finished until the Statesman and Caprice names were re-introduced in the VQ, and it continued until the introduction of the Gen III LS1 engines, which have a completely different engine number nomenclature.
As to why they did this... I have no idea! Perhaps the engines were balanced better to reflect the luxury level, or had different camshafts and/or tunes because of the additional weight, who knows?
Andrew