Yes you're right there Belladonna. Ovestin contains estriol as does - surprisingly enough - estriol cream! Estriol has greater affinity for beta-receptors (think I'm remembering correctly - my old link to the paper doesn't work any more so can't check this - but it's one or the other!) whereas estradiol binds more readily to alpha receptors, both of which are present in quantity in vagina, urethra and bladder. As far as I remember, as you say Belladonna it (estriol) has even less potential effect on the endometrium than local estradiol, but due to the receptors in the vagina the recommended dosage for vaginal atrophy is designed to have the same effect as estradiol ie Vagifem.
Hurdity x