There won't be any mechanical lock or such. The way some of the two cylinder boom sections work is that you simply provide hydraulic pressure to the tele system, and the section with the least resistance or largest piston will move first. With no load on, they will typically just go together.
Grove has some rigs with a roller that falls into a notch to activate the upper cylinder , but I don't think that national will have that. You would see it externally.
You just don't have it unbolted right. It shouldn't take that kind of pull to disassemble it. I'm not sure what you're trying to pull apart in your pictures, and what you have loose.
They will have holding valves in the cylinders, so even with the lines off, the extend side of the cylinder will be hydraulic locked (if the seals are still good). You have to pull the holding valve to remove the rod from the barrel.
The boom will just slide apart if you are going about it the right way. You still have something holding you up. Don't weld to the boom structure.