For sale: Two each, 21-tooth cogs, both for 1/2" x 1/8" chain. Both BSC (British) threaded. NOS: $22 shipped to US address. Light use but includes dirt and patina: $16 shipped to US address.
PLUS: I won't tell the list that YOU wanted something that will give this low a gear, even though we know your classic fixed gear bike has a 56 tooth chainring of an odd Spanish BCD for which you can't get a reasonable ring, and you will only use it for hill training on Sundays, and... :-)
IF you won't tell that I'm keeping another 21, and already have a 22, set aside for future infirmity.
You do NOT have to be AARP-eligible (age > 50?) to qualify, nor show evidence of past knee surgery.
Finally, please remember that you cannot convert a 21 tooth to inch-pitch by grinding off the alternate teeth, even though there were times that I would have liked a 10.5 tooth myself.
I would prefer to trade one of these for a 19 tooth of like quality.
As usual, if there more folks are interested than I have cogs, I will choose buyers 24 hr. after publication of the digest mode that contains this message.