I think it seems that in the final days of Autokraft, specifications of the so called Lightweights and everything else varied for all sorts of reasons. Could be regulatory issues, lack of stock or anything really.
Lets face it, we call the car the Mk IV, but Autokraft never registered that with the SMMT. That's why UK log books list the Manufacturer an AC, but the model is left blank. Upon request, you can have the make altered to AC Mk IV, but no model number. For the reason our cars do not appear on the DVLA Vehicle Information.
Looks quite easy for an unscrupulous person to rebadge the car, modify it and say its a Lightweight.