Right, after the IPO everyone is automatically converted to common but for the history of past rounds you need to look at the charter. The charter is publicly

Right, after the IPO everyone is automatically converted to common but for the history of past rounds you need to look at the charter. The charter is publicly available from the secretary of state wherever the corporation is incorporated. It will state how many shares of each series of preferred stock is authorized. Also, if there is price-based anti-dilution protection (typical in non-seed investments), the charter will state the "original issue price" of each series (i.e. the price per share when first sold). You can check the amount actually sold in the company's SEC Form D filings (also public) and work out the historical valuation that way. This doesn't account for things like notes converting at a discount, milestones, adjustments for down rounds, etc. But it is possible to get a rough idea of the valuation trajectory.