Update: Walter has translations for the Hebrew names above the "Dear Mother" and "Dear Father" inscriptions. The information sheds doubt on the Rabbi Spektor theory of Joseph's genealogy, though multiple questions remain to be answered. On Joseph's side, the Hebrew reads "Dear Father, Reb Yehuda, son of Abraham .... On Lena's side, it reads "Dear Mother, Rachel Leah, daughter of Abraham.... This is the first we have seen the names Yehuda and Rachel. Abraham does not match with the theory that Joseph was the grandchild of Rabbi Spektor. In that scenario, the father should have been Chaim or possibly Aryeh.
Somehow on my first several perusals of Harriet's trove of Rabinowitz materials I missed the most important page. Two of the pages were stuck together in my pile and until just now overlooked the following commemorative document recognizing the Yahrzeit for Lena and Joseph. What is so important about it is that it provides exact dates of death for our great grandfather and great grandmother, which will finally allow us to track down their New York City death certificates, which we expect will lead us to their burial sites and also provide parent names and other information.
Here is the Yahrzeit scroll. Click to enlarge. Hebrew translations welcome.