Have you seen the article 'Fine Structure of the Egg Chorion in Catocala (Lepidoptera, Erebidae)' by Kolesnichenko & Sidorov (2021)? I believe it to be very helpful for the identification of Catocala eggs. I'm currently on the look for C. promissa as well, since it's very rare here, but is increasingly becoming more common. Would be a blast to actually find it (but I probably won't). Unfortunately, C. fraxini isn't pictured in the article. But thankfully you already provided us with plenty reference material .