It's just a tiny hole, and a pin from the adapter fits into it.
I do believe I could modify a carrier - but I'd sure like to eyeball one first. I do have one for the newer style adapter (just the carrier, not the whole machine) but that one clips in a bit differently.
This is from the manual. My machine is actually a bit older than those that came with the looper - but from what I've researched, it should work.
Here is the shuttle holder I bought that came of the "newest" Domestic that did the chainstitch.
And the looper I have, which is the older style so it does not quite fit into the holder.
However I also have a "parts" Domestic machine, as it was cheaper to by the whole machine than to get a slide plate that mine was missing when I bought it.