When I press down the octave key using my hard rubber mouthpiece, doesn't matter which one, I have trouble hitting the high notes. When I use my metal mouthpiece, I can quite easily hit and play all the higher octaves. Both mouthpieces are #5 as well as a number 4 hard rubber Vandorens and Hite premiers. Can you help explain this to me? Thanks in advance as I am a beginner. Oh,and my Sax is a Conn 21M in mint condition.
SG

You might find experimenting with different strength reeds will cure this.
There is no international standard for numbering - the best you can say is that the larger the number the wider the tip opening. Even if the mouthpieces have the same number there will be other differences like the shape of the inside & length of the opening which will afffect how they play. Some mouthpieces are more suited to experienced players.
Try experimenting with different strengths of reed.

Yes - if you Conn is really in mint condition, than I think it has to be the reeds.

Just check though - is one mouthpiece larger than the other? and iff yes do you angle the neck slightly differently depending on which mouthpiece?
If yes it is just possible that the octave mechanism on the neck is not engaging properly?
I have an old Martin tenor that can get like this - it's a bit sensitive to getting the neck lined up.

This only affects some of the range of note in the octave.

a tech would be able to help you spot this if you have a freindly on on tap.