Maybe this is just my sad analytical mind at work, but I feel this should be more acurately 'estimatable'.
Possibly by estimating the number by manufacturer/series (possibly not accouting for how many kits completed or not, how many destroyed etc):-
(Σ Σ Σ (Series_High - Series_Low)) +#One offs
M S
Where M=Make/'Manufacturer' (e.g. Original, ERA, GTD, CAV, SPF, KVA, DRB, Tornado, MDA!)
S=Series (e.g. prototype, J car, MkI, MkII etc)
Series_High/Series_Low=the highest & lowest known chassis/serial numbers in each series (or just replace by total number if known).
or by considering how many are known in each region as this may be easier to estimate/certain individuals may have a good idea how many are in their area, then summing up over larger areas:-
Σ Σ Σ Σ
K C R
Where K=Continent,
C=Country,
R=Region (State/County etc)
There's probably not really that many countries (e.g. USA, UK, Australia, South Africa, NZ, Canada, France, Germany, Belgium, Japan, Sweden) that would need estimating to get a pretty good idea of the total number, and the remainder would probably not make too much difference. e.g. if USA=2000±200 then unless the 'rest ot the world' (not listed above) is significantly more than 50 it wouldn't change things greatly.
How many GT40s are there in Africa outside of South Africa?
Just my geeky thought for the day! I'm now going to go and get a life!!
Max
[PS just previewed it and the spacing is all messed up, so no comments about the layout (please)]