I suspect that it could just be something (toilet paper?) holding the gate open slightly, but if you want to have a gander at what things look like under there (assuming you have a rear bath model), this is what 1974 model looks like.
If it's just something stuck in the door, a more thorough cleaning of your tank might fix your problem.
The main difference is that in 1974 they added a grey tank. I can't say how much different the '73 is though. I'm going to guess that your black tant could be oriented 90° from this black tank.