Well this should do until Bud can chime in.. but Rare81 has many rare options for a 1981 corvette. First of all it has the 4-speed manual. It also has the radio delete. On top of this he has a charcoal exterior as well as interior, which are not common colors. All these (and I'm probebly forgetting a few) rare options made naming the car easy, Rare81.
1995 redvette- To directly answer your question; there was only one engine option available in 1981, the L81, rated at 190hp (there was also a california 305 cid engine, but was not an actual option you could pick from). Most '81 Corvettes came with the th-350 automatic transmission, so having a 4-speed at all is rare.
Bender- There is nothing particularly rare about 1981 Corvettes (almost 41,000 made), but Rare81 (Bud) has a rare optioned 1981 Corvette. The reason that there is a seprate forum and registry for the 1981 Corvette is basically because a bunch of fellow '81 owners got together and started it for appreciation of that year. There is nothing super-unique about this year. There are many year-specific corvette forums out there. Look for the 1979 corvette registry, its out there as well.
Hope this helps.
-Tatortot