1504 vs 1505?

[kmvhayes]

I've been looking for a Cross Century (original) 14k gold filled/rolled gold rolling ball pen to complete a set I've been assembling (I already have FP, BP, and pencil). I *thought* that this was the model 1504, which is sometimes labeled "soft tip", but which could accommodate different refill like gel etc. Then I saw model 1505, which appears exactly the same except it's labeled "Selectip", but also apparently accommodates different refills. Other than that, to my admittedly inexpert eyes, they seem identical. Any knowledge bombs to drop?

 

Also, I've noticed that some pens have a gold ring on the section, near where it connects to the barrel.  What's the difference? Should I look for a rollerball that matches my FP, which does have the metal ring?

 

Thanks for any help.

Kathleen