Well, there is a degree of the arms being "seized" to the shafts, if in fact "seized" is the correct term?
You can release the lock but the two pieces have been locked together long enough that they have a degree of bonding together.
It is the nature of the materials used to make them.
The only thing that you can do is squirt them with something like "Rust Buster", give it lots of time to penetrate, and continue to "wiggle" on the arms until they finally give up and let go.
Many parts like this never had the consideration of what they would be like to disassemble down the road 50 years later and in fairness, how could anyone determine that?
Probably the thing to do is once you have them apart and want to reassemble them, use something like "anti-seize" on the assembly?