The 1827 JR-2 Dime is the rarest variety of the year and, according to collector/researcher Jim Koenings, the seventh rarest of all Capped Bust Dime varieties.
This is an "eyeball" variety that can be attributed easily simply by looking at the top of the 1 in the denomination. On the JR-2 variety, the top of the 1 is flat as opposed to the pointed top that appears on all other 1827 Dime varieties.
Estimates of the known population of 1827 JR-2 Dimes ranges from 50-55 examples. Koenings illustrated 42 demonstrably different examples and listed nine additional examples without images (some of which may be duplications of the coins he illustrated). The finest example has an estimated grade of AU-53, but the majority of known examples grade only Fine or below. Several are new discoveries that showed up on eBay in the past few years.
Sources and/or recommended reading:
"1827 JR-2, The Most Underrated Capped Bust Dime Variety" by Jim Koenings
"Early United States Dimes 1796-1837" by David Davis, Russell Logan, Allen Lovejoy, John McCloseky, and William Subjack