Based on my experience with a few Gottlieb machines from the 50s (and I just checked a schematic from one to make sure), activating the coin switch (with coin drop) ultimately activates the subtract free play coil in the replay stepper during the start up routine (if there are replays available, otherwise there's a "zero free play" switch on the replay stepper that prevents this--or at least as far as I understand it!). In other words, if you have replays on the credit wheel when you drop a coin, you also lose a replay.

I'm not sure if machines from the 60s are like this or not, but you should be able to tell if this is normal operating procedure on Sweethearts by checking the schematic. I assume only newer (relative term) machines put replays on the credit wheel when a coins are dropped.