What is the primitive computing device that Swift uses?

Like the other men that had spoken before him, Swift reveals that he never trusted what Multivac was telling him to do. Multivac would tell Swift to fight here or there or even to wait, but Swift admitted that he just wasn't sure what to trust.

But I could never be certain that what Multivac seemed to say, it really did say; or what it really said, it really meant. I could never be certain.

Additionally, Swift knows that Multivac is a machine and isn't capable of bearing any kind of emotional weight that comes with hard decisions. Regardless of what Multivac orders, Swift is the one issuing that final order, so those decisions really affect him.

The horror of the responsibility of such decisions was unbearable and not even Multivac was sufficient to remove the weight.

He is very pleased to realize that the other two men also found problems with Multivac and frequently acted on their own intuition. Swift admits that he did the same as well. He also admits that he used a very old computing device to help him make the decisions involving one thing or another. Swift's "computer" is a coin. He would simply flip a coin to choose between two impossible choices.

With a faint smile of reminiscence, he flipped the coin he held. It glinted in the air as it spun and came down in Swift's outstretched palm. His hand closed over it and brought it down on the back of his left hand. His right hand remained in place, hiding the coin.
"Heads or tails, gentlemen?" said Swift.