This is an amusing thread - even if John's not amused at his loss. Sorry, John.

Apparently your ring-ding sprang (?) free and you lost the pin and U-shaped shackle that came at top of your block. But the block itself remained on the line (obviously). Right so far, John?
I'd refer to your "becket" as simply a "shackle" (pictured first by Tripp Gal). I've always understood the word "becket" refers to the lowest pin that hangs on extended cheeks below some blocks - as in Tripp Gal's right-most illustration. And here is the first I've ever seen reference to a becket-shackle~! (Sorry Tripp, but your Harken guy is REALLY confusing me.)
Shackles are available at West Marine in many sizes & shapes, examples are attached to the end of jib and main halyards. But as you've already discovered, these "shackles" are always larger than the little top shackles that come with most blocks. I'd just replace the entire block and toss the leftovers into my box of "spares." West Marine also sells a "kit" of stainless pieces like clevis pins, ring-dings and sundry stuff - also in my Spares box - alas, without any shackles. But you could just choose a particularly narrow shackle to suspend your block.
Also, you might request a copy of the Ronstan catalog through the mail. They offer a very broad range of their various bits & pieces that comprise their entire product line, including page after page of sheaves - every imagineable diameter of pulley, varying widths and styles and pin bores - maybe you'll find a mini-shackle in Ronstan's catalog.
Good luck!
