photocopy of contract...binding?

[tls1973]

CA - I bought a used car last week. When it came time to sign the contract, the dealership did not have any. They had me sign a "copy" of a contract and asked me to come back in about a week to sign an actual contract. One hour after leaving the dealership, I called them to find out if there was any way I could get the other car I had test drove because most of the features I remembered liking and thought I was getting were actually in the other car. The salesman had been rushing me along because he had "had a long day". I explained to the finance manager that I would normally never ask such a thing of them, but since I had not signed the "actual" contract yet, if there might be a chance...? He told me that there was no way because all of the paperwork had already been filed and sent to the DMV. I was really bummed out and I wish I had chosen the other car. Now they want me to come down there to sign the real contract...my question is, if the actual contract is still unsigned, why is it too late? And if it's too late, why do they need me to sign anything?