Adjusted Prices (1 for 4 consolidation in 2012 plus a few other adjustments over the years).
If I couldn’t calculate adjusted cost base from source documents in your sort of situation, I would adopt the closing adjusted price above. I suspect the share registry could confirm and document the above prices for you. With that documentation in place, if in the unlikely situation that the ATO has better information and wants to correct you, I suspect they would do so without issuing a penalty as you have made best attempt estimate under the circumstances.
That’s just what I would do as a complete amateur.