I found it! Very much after the fact, but I finally found the part that is missing from my 925D serger. This part (or lack of the part, rather) gave me much angst when I discovered it was missing from my ebay purchase.
I was just browsing the website reviews on Patternreview.com, and came across a review for the site www.sewingstyle.com. I clicked the link without reading the review, and scrolled down the page. Then, on a whim, I decided to search the sewing parts - selecting the "loopers" category. Lo and behold - on the last page - the item that has evaded me for months - with a picture even!! The Brother guide calls it a "stitch finger". This site calls it a "stitch tongue". Doesn't matter - never have I been so happy to see a machine PART!
Okay, just wanted to say that. I'll be ordering it. Just too lazy to get up and go find my purse right now...
STOP! DON'T GO TO THAT SITE! I JUST READ THE REVIEWS, AND THEY ARE MOSTLY NEGATIVE! Hmmm, who knew my laziness would pay off? I think I'll hold off on ordering the part rather than give up my credit card info and possibly have fraud issues to deal with later. So, in spite of the fact that the reviews are negative, PatternReview has proven to be a useful source again! Whew!