The serial # will be stamped on the body under the battery. You say you tracked down the original selling dealer- they should have a "line sheet" on the unit which would list everything the truck came with from new- that will answer the question about a fast lift option.

As far as what cylinder is in it now, the outer mast channel will have a number stamped in it facing the front of the machine- you'll have to raise the carriage to see it. It may be on either the left or right, usually about 3' up from the ground. That will tell what upright/ cylinder is installed presently.