Famous-Maker Animal Slippers - $10.99,'' reads the orange cardboard sign. It is tacked to a wooden stick and stuck in a bin of what are indeed famous-maker animal slippers at $10.99 - and are being pawed over by curious would-be buyers.You know right off that this is not Saks Fifth Avenue. There is no bouncy carpeting underneath. No hushed ''May I help you?'' from tastefully dressed sales personnel. No clever display of Italian silk sport jackets gently spotlighted from above like fine art objects. Instead, there is linoleum underfoot, fluorescent lights overhead, and a wealth of humanity at your elbow.Is this any way to shop?You bet it is, say the customers of Filene's Basement, the oldest and best-known bargain-basement store in the country. And they've got the numbers to prove it - not the least of which is the 75th anniversary which ''the basement,'' as the locals call it, has just celebrated, cake and all.Filling the bottom two floors of Filene's department store in the heart of Boston, the basement offers top-quality goods at discount prices. However rough and tumble, the basement is, in fact, a miniature department store.There are no dressing rooms. Clothing hangs on metal racks or is stuffed into wooden bins. But everything from children's clothes to wedding dresses to luggage is for sale from 30 to 70 percent off, with more markdowns at future dates indicated on the price tags in what Filene's calls its ''automatic markdown system.'' After 30 days, all unsold items are donated to charity. One of the recipients, Goodwill Industries, even named the basement ''Contributor of the Year'' in 1982 because of its largess.With that retailing philosophy - and with merchandise that embraces Schiaparelli gowns, Stanley Blacker blazers, and L. L. Bean loafers - the store has become an institution in a city whose Spartan sartorial standards run more to button-down shirts and aging tweed blazers. ''It's a treasure-hunt atmosphere,'' says Filene's Basement vice-president Melvin Johan. ''It's a very exciting shopping experience - and that excitement is created by the traffic.''That ''traffic,'' coupled with a lack of traditional department-store amenities, has caused Boston shoppers to fall into two camps: those who insist they ''wouldn't shop anywhere else'' and those who just as staunchly maintain they ''never shop the basement.''It is admittedly not the spot for the fainthearted. But store officials insist that their customers are ''sophisticated'' and that they know value when they see it.The Boston store's reputation, Mr. Johan says, ''is based on the fact that customers truly find value bargains every time they come into our store.''While some observers charge that the basement isn't what it used to be - with the new suburban stores and competitors cutting into the business - the mystique of the original basement store somehow survives. ''We see people come up from New York just to shop the basement,'' a tourist official says.And stories are rampant. Including the one about two customers who tore a cashmere sweater in half. And the woman who draped her own dress over a rack while trying on clothes - and watched another shopper try to buy it. And the time in the '50s when the Boston Red Sox arrived en masse at the store to buy raincoats.Whatever the allure, retailers and market researchers confirm that Filene's Basement represents that part of the retail industry growing most rapidly. Analysts suggest that already the phenomenon referred to within the industry as ''off pricing'' has captured 6 percent of the American apparel market - a figure that many expect to double by 1990.It is apparently the recipe for success. Originating in the early 20th century as the unique brainchild of the Filene brothers, the merchandising concept of reducing prices at regular intervals took 10 years to turn a profit. Since then, growth has been consistent. Branch stores have sprung up - 11 so far in suburban Boston and New York locations, with three more on the way - and the downtown store has continued to increase sales.Company officials won't disclose how much the football-field-size operation in downtown Boston grosses during the year, but operations manager Andy Psilopoulos says that ''at least a thousand people are on the floor at any one time.'' Retail analysts and marketers talk less guardedly. They say lean economic times for retailers - 1982 was one of the worst years in recent memory - have fostered changes within the industry, most notably the growth of the off-pricing strategy that Filene's Basement pioneered 75 years ago. ''Since the mid-1970s, the discount and off-price stores have grown tremendously,'' says Leonard Barry, director for the Center of Retail Studies at Texas A&M University. ''The high inflation of yesterday and the high unemployment today, coupled with continued high interest rates, have caused millions of consumers to become avid get-my-money's-worth shoppers. And off-pricing stores cater to that.''Such stores generally discount the cost of the items they sell by up to 70 percent. They can offer the low prices because they buy the merchandise - usually manufacturers' overruns, end-of-season surpluses, and seconds - at below-normal wholesale prices. ''Even though the price the customer pays is extremely low, the off-price store is generally grossing as much as a regular store because their markup is roughly the same,'' says one retail expert.Such retailing has been a boon to apparel manufacturers, who often create deliberate factory overruns, knowing that an off-price store will snap up extra merchandise - often for cash.But many fashion designers and regular department-store officials are less pleased with the trend. Many feel it is cutting into the regular retailing business. ''It's causing a lot of concern on the part of traditional stores,'' says Kurt Barnard, executive director of the Federation of Apparel Manufacturers. ''They're losing a lot of business to the off-pricers.''Some traditional stores are fighting to keep their exclusivity by emphasizing in-store brand names - or by cutting their own sale prices. Still others say that it is easier to join the phenomenon than to fight it. For instance, Associated Dry Goods, owner of Lord & Taylor stores, recently purchased Loehmann's, a highly successful chain of 64 off-price stores.Is Filene's Basement thinking of going nationwide?''We'll never discount an opportunity,'' Mr. Johan insists wryly.