A couple of years ago I got interested in a particular Mi Yodeya question concerning the so-called "non-verse" in Selichot, "זכור ה' חיבת ירושלים אהבת ציון אל תשכח לנצח" (link; link to my answer). Yep, most people probably aren't aware of this, because it's nestled among real verses, but this sentence does not appear anywhere in Tanach. You can check out my answer to see the information I gathered at the time. Not long after that I rewrote the answer as a Hebrew essay and published it at my yeshiva's monthly journal.
Since then, every now and then I've tried looking into the topic a teeny bit more. I soon became aware of a paper published in 1906 by Rabbi M. J. Abrahams (here, pp. 109-113) which was the scholarly publication of a leaf from a selichot siddur from 12th century England found in Pembroke College! In that paper, Rabbi Abrahams noted that two leaves had been found, but he had only managed to read one of the leaves, the better preserved, and so did not publish the second one. I was excited to learn that the leaf he published featured a section of text that immediately followed the "זכור verses" section, and figured there was a good chance that the other leaf would feature that verse.
I contacted the Pembroke Library to see whether they had plans to digitize the relevant manuscript. I was told that it was indeed on the waiting list. I was also informed that the second leaf was also part of the same siddur and immediately preceded the first leaf. And so I waited in excitement at the prospect of seeing a rare medieval British version of the זכור verse. Fast forward a couple of years, and Pembroke still hasn't digitized that text (I last checked around Pesach-time). However, a short while I ago it occurred to me that perhaps the National Library of Israel had a microfilm copy of that manuscript. So I checked, and indeed they did have a microfilm of both leaves (!). Before I started planning a time to visit the NLI, I noticed the bibliographical source they mentioned: Not Abrahams, but one Collette Sirat, who had apparently republished the first leaf at least, perhaps both! I quickly found Sirat's paper, and yeah, I was pleased to discover that she had both republished the first leaf with a new suggested reading, published the second leaf, and also printed images of the two leaves!
Unfortunately, that's where my excitement ended. While the second leaf was, indeed, the immediate predecessor, the relevant section would have probably been in the top part of the first leaf, which had been torn many centuries ago (the leaves were found being used as part of the cover of a Latin gloss on Sefer Yesha'ayahu).
So, there's the end of that mystery. Unless the torn part of the first leaf is found, we'll never know what was the version of the זכור verse in that particular siddur.
