Here is the info I have on parts numbers.
I wouldn't say that they won't interchange but there is something different on the shafts that might require you to use other parts.
13127-1118 SHAFT-TRANSMISSION IN
1985 KX500-B1 KX500 TRANSMISSION
13128-1042 SHAFT-TRANSMISSION OU
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KX500-B1 KX500 TRANSMISSION
13129-1132 4TH GEAR,OUTPUT,21T
1979 KDX400-A1 KDX400 TRANSMISSION
1980 KDX400-A2 KDX400 TRANSMISSION
1980 KX420-A1 KX420 TRANSMISSION
1981 KX420-A2 KX420 TRANSMISSION
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KX500-B1 KX500 TRANSMISSION
13129-1133 TOP GEAR,OUTPUT,19T
1979 KDX400-A1 KDX400 TRANSMISSION
1980 KDX400-A2 KDX400 TRANSMISSION
1980 KX420-A1 KX420 TRANSMISSION
1981 KDX420-B1 KDX420 TRANSMISSION
1981 KX420-A2 KX420 TRANSMISSION
1982 KDX450-A1 KDX450 TRANSMISSION
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KX500-B1 KX500 TRANSMISSION
1986 KX500-B2 KX500 TRANSMISSION
1987 KX500-C1 KX500 TRANSMISSION
13129-1180 LOW GEAR,OUTPUT,32T
1980 KX420-A1 KX420 TRANSMISSION
1981 KX420-A2 KX420 TRANSMISSION
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KX500-B1 KX500 TRANSMISSION
1986 KX500-B2 KX500 TRANSMISSION
1987 KX500-C1 KX500 TRANSMISSION
1988 KX500-D1 KX500 TRANSMISSION
1989 KX500-E1 KX500 TRANSMISSION
1990 KX500-E2 KX500 TRANSMISSION
1991 KX500-E3 KX500 TRANSMISSION
1992 KX500-E4 KX500 TRANSMISSION
1993 KX500-E5 KX500 TRANSMISSION
1994 KX500-E6 KX500 TRANSMISSION
1995 KX500-E7 KX500 TRANSMISSION
1996 KX500-E8 KX500 TRANSMISSION
1997 KX500-E9 KX500 TRANSMISSION
1998 KX500-E10 KX500 TRANSMISSION
1999 KX500-E11 KX500 TRANSMISSION
2000 KX500-E12 KX500 TRANSMISSION
2001 KX500-E13 KX500 TRANSMISSION
2002 KX500-E14 KX500 TRANSMISSION
2003 KX500-E15 KX500 TRANSMISSION
2004 KX500-E16 KX500 TRANSMISSION
13129-1407 GEAR,INPUT 2ND.22T
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KX500-B1 KX500 TRANSMISSION
13129-1408 GEAR,INPUT 3RD,22T
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KX500-B1 KX500 TRANSMISSION
13129-1409 GEAR,INPUT 4TH,22T
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KX500-B1 KX500 TRANSMISSION
13129-1410 GEAR,INPUT TOP,24T
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KL600-A1 KL600 TRANSMISSION
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KL600-B1 KL600 TRANSMISSION
1985 KX500-B1 KX500 TRANSMISSION
1986 KL600-B2 KL600 TRANSMISSION
1987 KL650-A1 KLR650 TRANSMISSION
1988 KL650-A2 KLR650 TRANSMISSION
1989 KL650-A3 KLR650 TRANSMISSION
1990 KL650-A4 KLR650 TRANSMISSION
1990 KL650-B2 Tengai TRANSMISSION
1991 KL650-A5 KLR650 TRANSMISSION
1991 KL650-B3 Tengai TRANSMISSION
1992 KL650-A6 KLR650 TRANSMISSION
1993 KL650-A7 KLR650 TRANSMISSION
1994 KL650-A8 KLR650 TRANSMISSION
1995 KL650-A9 KLR650 TRANSMISSION
1996 KL650-A10 KLR650 TRANSMISSION (~KL650AE032209)
13129-1622 GEAR-OUTPUT 2ND,32T
1980 KX420-A1 KX420 TRANSMISSION
1981 KX420-A2 KX420 TRANSMISSION
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KX500-B1 KX500 TRANSMISSION
13129-1679 GEAR-TRANSMISSION SPU
1979 KDX400-A1 KDX400 TRANSMISSION
1980 KDX400-A2 KDX400 TRANSMISSION
1980 KX420-A1 KX420 TRANSMISSION
1981 KX420-A2 KX420 TRANSMISSION
1983 KX500-A1 KX500 TRANSMISSION ('83 A1)
1983 KX500-A1 KX500 TRANSMISSION ('84 A2)
1984 KX500-A2 KX500 TRANSMISSION ('83 A1)
1984 KX500-A2 KX500 TRANSMISSION ('84 A2)
1985 KX500-B1 KX500 TRANSMISSION