At the start there were only twelve.
...but it can be computed back from there.
You claim an industrial average can be extrapolated back 96 years, decades prior to the industrial revolution?
Ludicrous.
Given the proper source material, i. e. the stock prices as published daily by the exchange and reported by the press it should be possible to compute any index back to the origin of the market. Not saying I’d like to do it but if some bright boy or girl working for his masters wanted to try it I’m sure they could.
Ludicrous though it may be, I’ve seen the chart published in several books.
But far be it from me to introduce facts, observations or opinions that contradict your obviously superior preconceptions.