Values
4XX	Byte 0	Byte 1	Byte 2	Byte 3
	1st Position	2nd Position	3rd Position	4th Position
	n	n	n	n
			a	a
			e	b
	3rd Position:	a= pre-AACR2 heading (linking reference)
		e= earlier AACR2 heading
	4th Position:	a= reference not made (displayed)
		b= cataloger-generated reference with 664
		(may find, but never make)
5XX	a	n	n	a
	b			c
	n			n
	1st Position:	a= earlier heading
		b= later heading
		n= default
	4th Position:	a= reference not made (displayed)
		(used in only one case)
		c= cataloger-generated reference with 663
		(used with pseudonyms only)