I've never seen a 20" barrel marked 'full'. Any short barrel I've seen, before the advent of screw-in chokes, has been cylinder bored. I'd say somebody was busy with a hacksaw.
You never seen one my friend because remington never made them in 12ga.
To the Op sounds like a cut off job but to be 100% check the choke diameter at the end of the barrel. I would say there is none and straight bore
AX just tells you it was made March of 73 nothing more
Cheers
The Remington Date Codes are always stamped into the barrel's lower side, near the rear - just ahead of the receiver.
The first letter of the Code is the month of manufacture, followed by one or two letters which are the year of manufacture.
As noted above, for shotguns with removeable barrels, the code will be valid for the manufacture of the barrel; maybe for the receiver, as barrels do get switched around.
Month Codes: [first letter]
B - L - A - C - K - P - O - W - D - E - R - X
1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12
Year:______Code: [second (and third * letters]
1950____*___ WW
1951____*___ XX
1952____*___ YY
1953____*___ ZZ
1954_______ A
1955_______ B
1956_______ C
1957_______ D
1958_______ E
1959_______ F
1960_______ G
1961_______ H
1962_______ J
1963_______ K
1964_______ L
1965_______ M
1966_______ N
1967_______ P
1968_______ R
1969_______ S
1970_______ T
1971_______ U
1972_______ W
1973_______ X
1974_______ Y
1975_______ Z
1976_______ I
1977_______ O
1978_______ Q
1979_______ V
1980_______ A
1981_______ B
1982_______ C
1983_______ D
1984_______ E
1985_______ F
1986_______ G
1987_______ H
1988_______ I
1989_______ J
1990_______ K
1991_______ L
1992_______ M
1993_______ N
1994_______ O
1995_______ P
1996_______ Q
1997_______ R
1998_______ S
1999_______ T
2000_______ U
2001_______ W
2002_______ X
As maybe seen, the year code letters duplicate; some knowledge of when the model was introduced should resolve the actual year of manufacture.