Within the six-story building in Hawthorne, Calif., the second floor is sealed off to any unauthorized visitor. A guard controls a turnstile at the front entrance while a TV viewer keeps a constant scan on the rear access.

In the plant at Stamford, Conn., inventors and designers work in a sealed-off area. Only a select, screened few (at least of the status of vice president in the company hierarchy) have keys to gain entrance. Later, designs are translated into products within the security of locked rooms.

"Some people in the industry act as if they are inventing the atomic bomb," says William F. Dohrmann III, who tries not to be "paranoid" about security at the Parker Brothers factory in Salem, Mass. (although he admits the swinging doors were installed so that a casual visitor can't look back into the research and development offices and lab).

Behind all this security, grown men and women - talented designers and inventors - are playing a deadly serious game with toys for high Monopoly stakes running into millions of dollars.

It's not all fun and games when it comes to the highly-competitive toy business. An advance leak on the design of an electronic game or baby doll can mean the loss of hundreds of thousands of dollars in tool-up costs before even starting to count anticipated profits.

"I live in deadly terror that I'll get caught with 100,000 toys in the warehouse with someone beating us to the market," says Millens Taft, Senior vice president for research at Milton Bradley Co. in Springfield, Mass.

All a competitor has to do is bring out a slightly different model at a slightly lower price and he can steal the market. If the name is similar enough to be confused, he can get a free ride on TV and advertising commercials.

It isn't without reason that toy manufacturing is known as a "knockoff" business for its stiff competition on marketing products.

Until the toy fair in New York City in February, toy manufacturers guard their bright new ideas with the real of Pentagon security officers.

"Espionage in the toy business is not over but more sophisticated and indirect," explains Juergen Rathgeber, vice president for product development for the Marx Co.

Designers and inventors frequently switch companies, he points out. Like the CIA, Marx requires employes to sign an agreement of confidentiality that opens them to liability law suits if secrets are leaked.

Then there is the old-boy spy network that flourishes on gossip and rumor. In New England triangle of old-line toy manufacturers, the same salesmen and suppliers can serve several companies, Dohrmann, of Parker Brothers, points out.

Dohrmann, like most other toy design chiefs, doesn't work much with freelancers. For one thing, they can innocently leak secrets - in going from company to company to peddle their idea. For another they're always claiming a toy manufacturer has stolen their idea.

"We always can pull out blueprints to show that we have been working on the idea four years earlier but it takes a lot of legal time," Beverly Stinnett, Mattel's consumer adviser, points out.

"A small company can seize an idea design if there is a leak and rush it into production in the Orient with a similar name but not the quality. It's a knockoff business.

The "knockoff" isn't quite so easy to pull off these days with toys getting more sophisticated and complicated. By the time of the February toy fair, manufacturers feel secure in unveiling new products because it generally is too late for a competitor to rush into the market.

It may not yet be Christmas 1977, but Dohrmann, Taft and other design chiefs already are spending most of their time on products for the 1979 market.

One reason for the need for design secrecy rivaling a new weapon is that most toys can't be patented.

"It's worth its weight in gold to have a patentable toy," says Milton Bradley's Taft.

To get a patent, a toy generally has to have some kind of a mechanical device. At Ideal Toy this year, one of the patents is for TCR, which stands for total-control racing. Julius Irving, senior vice president for research and development, says the patent covers how the car steers on a slotless track. A patent is pending on a third "jam" car.

A trademark can be used to protect a name. In England, both Milton Bradley and John Waddington came up with the name "black box" for two completely different toys. They had to work out an amicable agreement on the use of the name.

As he works on 40 candidates already up for consideration for the 1979 toy line for Parker Brothers (the 42-year-old board game of Monopoly still accounts for 13 per cent of the company's sales), Dohrmann says:

"I would give my eye teeth to get a line on what the others are going - short of breaking the law."