"Above"/"below" before/after a noun

There is not really any difference between reference above and above reference, but some publishers may prefer one or the other. Below reference will be rare, if it is found at all.

On reading the page RegDwight ΒВB linked and dictionary entries for above and below, I have come to realise that above is listed as an adjective as well as a preposition (and adverb, too), but below is only a preposition (and adverb), so only (1), (2) and (3) are correct.

"above" is an adverb and can't be used as an attributive adjective before a noun. So "the above gun" is wrong grammar.

"above" is also a preposition. See OALD:

http://www.oxfordlearnersdictionaries.com/definition/english/above_1

There is even an entry about above as adjective. One special case, in letters, e.g. "the above person". As this is short for the above mentioned person, I would not say that it is adequate to lable "above" simply as adjective. At least a remark would be necessary, "above, attributive adj., limited use".