Lightly used Wolf Tooth cassette range extender. Intended for SRAM 10 speed drivetrains.

The premise is to remove 2 cogs from the middle of the cassette to replace with the 16 tooth and then the big 42 on the end. I'll say it worked "good" in that it did extend the life of my 10 speed stuff but the gaps in tooth count made for some big jumps between ratios.

That derailleur met its unfortunate end from a stick and I upgraded so the range extender is no longer needed.