Thanks to some wonderful tatting friends and sleuths, I now have the pattern I've been looking for! I feel so lucky that I received so many responses regarding the cross pattern. I'm pretty sure that the original bookmark I received was from this issue of The Workbasket, because it was given to me in 1967 or 1968, too early for the Rebecca Jones book or the Becky Dempsey version.
Thank you, Patti! This is the April 1955 issue, and the pattern I've been looking for is right there on page 6!
Lucky me, I also have the December 1958 issue on its way to me! That's the issue I was originally looking for, but I have found that the cross in that issue is slightly different. I'm really going to have fun tatting both of these pretty patterns this summer. Now to get my tassel skills up to snuff!