Cable's Arts & Entertainment network will soon be showcasing a major archaeological find: 51 episodes of The Avengers never shown in the United States. The sometimes surreal British spy series began its run in 1961 but wasn't imported into the U.S. until 1966. For reasons no one has yet figured out, the show's 1962 and 1963 seasons never made the trip to this country until now. ''The original production company in England was looking around in their archives and they found that they had these old episodes,'' says Michael Katz, A&E's director of programming. ''We lucked out and were able to buy them.'' The early episodes star Honor Blackman (Pussy Galore in Goldfinger) as the pre-Diana Rigg sidekick to secret agent Jonathan Steed (Patrick Macnee, above). ''Blackman was the first to wear leather pants on the show,'' Katz says. ''She's sort of a cross between Margaret Mead and a sex bomb.'' The old episodes begin airing Oct. 1, but some fans may be a bit disappointed: ''The production quality is pretty poor,'' Katz admits. ''It's not very sophisticated. There aren't many exterior shots.'' Still, ''the old shows hold up pretty well. They have the same tongue-in-cheek attitude of the later episodes. It was a sophisticated series even way back then.''