Wasn't the M code reserved for Hemi/racing cars, or am I remembering wrong?
Up through 1965, a Chrysler corp. VIN only distinguished between a 6 or an 8, but starting in '66, it indicated the installed engine type. Between '66 and '80, it was the 5th digit, from 1980 on, it's the 8th digit.
In regard to the "M", it was a special order V-8 engine in 1967, in 1969 it denoted a 440-6, from '72 through '76 it was a 440-2 and in '81 '82 it was a 318-4
* edit *
Tought I'd toss the full engine code list in there for the heck of it!
A
170-1 1966-69
1.7-2 1978-82 (VW)
1.6-2 1983-86 (Peugeot)
2.2 T2 1987-89 (including '89 TC, 5th VIN digit)
2.2 T3 1991-93
B
225-1 1966-69
198-1 1970-74
2.2-2 1981-82
1.7-2 1983
C
Special Order 6 cyl 1966-69
225-1 1970-80
2.2-2 1983-87
2.2 T4 1990
D
273 1966 (2 or 4 bbl)
273-2 1967-69
225-2 1977-80
2.6-2 1981-82
2.2 EFI 1985-94
E
318-2 1966
273-4 1967
Special order 6 cyl 1970-80
225-1 1981-82
2.2 T1 1984-88
488 V10 1992-
F
361 1966 (2 or 4 bbl)
318-2 1967-69
225-1 HD 1981-82
2.2-2 HP 1984-86
3.5 1993-
G
383 1966 (2 or 4 bbl)
383-2 1967-69
318-2 1970-80
225-2 1981-82
2.6-2 1983
H
426 Hemi 1966
383-4 HP 1967-69
340-4 1970-73
318-4 1978-80
225-1 1983
J
440 1966
426 Hemi 1967-69
340-6 1970-71
360-4 1974-79
225-1 HD 1983
2.5 T1 1989-91
K
Special Order V8 1966
440-4 1967-69
360-2 1971-80
318-2 1981-82
225-2 1983
2.5 EFI 1986-95
L
440-4 HP 1967-69
383-2 1970-71
360-4 HP 1974-78
318-2 HD 1981-82
M
Special Order V8 1967
440-6 1969
400-2 1972-76
318-4 1981-82
N
383-4 1970-71
400-4 1974-78
318-4 HD 1981-82
P
340-4 1968-69
400-4 1972-73
400-4 HP 1974-78
318-2 1983-89
R
426 Hemi 1970-71
318-4 1983-87
2.2 T 1989-90 TC only--5th digit (DOHC Maserati head)
2.0 1990- DSM Eclipse/Talon only
3.3 1990- non-LH
S
318-4 Police 1983-89
3.0 1990-91 TC code ONLY--5th digit (same Mitsu 3.0)
T
440-4 1970-77
1.8 1990- DSM Eclipse ONLY
3.3 1993- LH
U
440-4 HP 1970-77
2.0 T 1990- DSM Eclipse/Talon ONLY
3.0 1990-
V
440-6 1970-72
X
2.2 LPG 1987
Z
Special Order V8 1970-78
318 LPG 1987