Replacing my distributor first time

Just bought an 89 RamCharger with the 318 and need to replace the stock distributor. My question is as follows. Am I to assume I am removing distributor gear off the old part and then installing it onto the new distributor shaft? Ever mopar distributor no matter what brand on line shows no gear on the shaft. But when I notice Chevy or ford distributors they have a new gear already on the shaft. The mopar distributors seem to have a flat part at the end of the shaft as if I'm supposed to put a gear on it. Am I missing something