|Freehub Wheel (what my cassette fits on)|
|Freewheel Hub (what I ordered anew, arrhh!)|
More detailed schematics of these two types are available at Wikipedia. Unfortunately, my former lack of knowledge left me with the wrong wheel. Further investigation also taught me that I am likely to need spacers to properly fit my old 7-speed cassette to a new hub because the latter are generally designed for cassettes with 8-10 gears. Those of us who are satisfied with merely 7 gears will need to cope/adapt. Anyway, this additional wrinkle made it likely (to me, at least) that additional online shopping for spacers could lead to more error in parts selection. Therefore, this morning I opted for the bike shop, where my Trek sits being repaired in a well-stocked shop.