On the captains chairs on my bridge the bolsters are all stacked and held together by two long all thread rods from the top head rest to the base. Two acorn nuts on the bottom of the seat. The seat bottom bolster is held in with Velcro. May be similar. Pic might help.
Well they are definitely different than my seats. Your backs appear to be solid one piece. The bottoms may still be held in with Velcro. You may find that its cheaper to just replace them with after market unless you are going with a custom design to match the rest of the boat.
My seats lift straight off the post (with a little wiggling). There is a 4" piece of plastic in between the post and the inverted cup on the seat that keeps everything tight.