I was reading this for a while and am throwing my two cent worth of thought in to this conversation: AKL 1333, under your definition think about that: AK 1005 (no L ) has for sure a much closer suspension set up then any lightweight (whatever build date), because in 1982 there was not that extensive use of donation parts coming from Ford, Volvo, Renault or BMW or other cars as very early cars there is a one to one swap from MK III components having for example Girling brakes and all suspension parts etc. from MK III. Now is this car then even of more value then a lightweight? Is it even more real? It has short nose as well? The engine and gear box package was added in US as done with the original Shelby cars having engine (carb) and T10 gear box and even from the fabrication dates correct to get a 1966 registration? Does this all increase the value of a car - even so there where only the "famous" 12 cars build with a kind of special history?
Such little details as of having heater vents or not - I don't believe should add any great selling point to the lightweights. Now there are differences so is the door hinge with side bar, the rollbar and the location of the fuel filler - including some difference on the rear fire wall.
But as said by previous comments, all is possible to get modified and made to what people like, and not to hard as well. We all kind of take advantage of a big kit car community and their parts, making such modifications quite simple and inexpensive.
And we should not forget, that people tent to modify these cars to what they like - a Cobra seams by nature something people take their hands on - from past until today. There are many examples of such modifications on this forum where people show with bride what they have done to their original MK IV cars. And they receive applause from all of you for their great work. Now, have they added value or did they destroy value? Not sure to have the answer, but in Germany at least people with strong modifications to MK III level seam to make better prices then "original" MK IV's, even on lightweight cars. And the power level of their engines seam to sell as well. That's also a selling point for replicas today with asking prices far higher then a Mk IV would bring, even in whatever lightweight specification and build year. And it looks like that most have the desire to modify their cars to get as close as possible to MK III specification - it looks like everybody has the desire to get a car look a like MK III. Are they butting their cars then to the same level as a kit car? For sure it seams like that they sell better then not done, at least in Germany.
For sure in performance with the later build lightweight cars there is a huge difference to a real MK III performance - these lightweight cars without modifications get busted by almost any other car with a little sport trim or a sedan with a decent diesel engine, even in acceleration.
And even the earlier lightweights with their 345hp or so out of a 5 liter engine are not really getting to the performance levels a 427 SC is going to - that's I guess why such cars have been strongly modified even right after purchase at Autokraft. Is this increasing or decreasing value to these cars?
Now back to the initial question, what makes a MK IV lightweight more special? Was it the price people where willing to pay for it in first instance? Are the first 12 cars build then even more special as being most close to 1960 specifications?