According to Apple the various MacBooks won't run with smaller power supplies than was originally shipped. So a 60W shouldn't work with my 17" which came with an 85W supply. The power supply bricks send DC over 2 wires to the MagSafe connector - nothing fancy there. But inside the connector there is programming that identifies the wattage (among other things) to the computer's SMC, which makes the decision to use that supply or not. That is the purpose of the small 5th pin in the middle of the connector: it is a serial
interface (also used to turn on the MagSafe's LED). I have no idea where the cloners are sourcing their connectors; in the early years Apple refused to sell or license
them to anyone, which had a few long-term Apple partners more than a little upset. Perhaps the patents have expired - I just don't know. But the difference between the computer accepting the supply or not is inside the MagSafe connector, not the power brick. And the current
85W MagSafe will not work with my MBP because Apple didn't offer an update so my SMC will be happy with the new info in the connector.
Mark, you are very lucky to get the cord. It seems most of the MagSafe era power supplies fail because of either a frayed cord near the brick or problems in the connector, not so much a problem with the power itself. It is the connector that is unique and valuable - there is no shortage of 18V DC power supplies.