The back of x-01 power armor is just a softshell mirelurk

You get a follow if you can guess where thar armor is in the picture

The Castle?

Yes

Heck yes!

That makes sense since that specific model of armor was made after the bombs dropped.

I suppose. If the lore's right, X-01 Power Armor was made by the Enclave to improve upon other models by adding more armor and more maneuverability compared to others.