Didn't realize that I'd been away from this thread for a month, and I apologize for my rudeness at not getting back to those who were kind enough to help. In any case, should anyone still be reading this thread, the item I pictured in an earlier post was indeed a lens adaptor. B&H shipped it to me in a box labeled "retaining ring." They apologized and offered a refund, then (as I'm fortunate enough for these purposes to live in NY) I went up to the B&H used equipment department and bought after inspection a true copal 0 retaining ring for about $10, came home and attached the lens to the lensboard without difficulty. Should have done this in the first place, of course, and I'm grateful for the help I received here. I do have another question about shooting with the Crown Graphic, but I'll do this in a different post. Thanks again.

Wow, I made the same mistake for about the third time, posting a new topic when I meant to post a reply. Anyway, the post above here was a follow up to a thread below about difficulty I had with a retaining ring (that wasn't a retaining ring after all).

Great! I'm glad you survived ...