I have one big frustration with freewheels -- the availability of 26T, which is my perennial favorite low-gear cog, because it works with a short cage derailleur and, with a 42T chainring (144mm BCD minimum), it gives me a low enough bottom gear to handle any of my local hills.
Back in the 1960s and 1970s, 13-26 and 14-26 freewheels were very common, but they seem to have disappeared on the open market, and all of the new ones are 28s, instead.
Fortunately, I have my own pretty well-supplied DIY freewheel cog (and chainring) board at home.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069