It is a mixture of the sodium salts of estrogen conjugates found in horses, such as estrone sulfate and equilin sulfate. CEEs are available in the form of both natural preparations manufactured from the urine of pregnant mares and fully synthetic replications of the natural preparations.
This appears not to be entirely correct: https://en.wikipedia.org/wiki/Conjugated_estrogens
I think you may have misunderstood my comment. I was speaking to this part of yours:
In that what we use now isn’t really “human” oestrogen, because it’s synthetic, and oestrodiol is pretty much universal among mammalian species.
True. I said human estrogen because the ones found in premarin aren’t found in humans, while the new bioidentical ones are.