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 90į from this black tank.
ďCourage is being scared to death, but saddling up anyway.Ē