I was trying to find out what engine I have on my buggy and came across this:
Engine ID Codes
VW stamped all type 1's and pre '72 type 2's on the right case half just below the generator pedestal.
The first and sometimes the second digit/letter denotes the basic engine group.
So the best you can do is narrow down the engine to a group.
Here are the groups
Type 1
Code Year Engine Remarks
4 - 60 - 1200 - 40 HP
5 - 61 - 1200 - 40 HP
6 - 62 - 1200 - 40 HP
7 - 63 - 1200 - 40 HP
8 - 64 - 1200 - 40 HP
9 - 65 - 1200 - 40 HP
FO - 66 - 1300 - 8mm oil pump studs
HO/TO- 67 - 1500 - 8mm oil pump studs
H5 - 68-69 - 1500 - 8mm oil pump studs
B6 - 70 - 1600 - Dual Relief - single port
AE - 71-72 - 1600 - Dual Relief - Dual Port
AH - 73-74 - 1600 - 8mm Head studs w/case savers
AJ - 75 - 1600 - Fuel injected, no fuel pump mount.

Type 2
Code Year Engine Remarks
Numerals 61-63 - 1200 - See type 1 40 HP
O - 64 - 1500 - No Cam bearings
H - 65 - 1500
HO - 66-67 - 1500 - No Cross member holes
B5 - 68-69 - 1600 - 8mm oil pump studs, single port
B5 - 70 - 1600 - Dual Relief
AE - 71 - 1600 - W/cross member holes
Late Bus
CB - 72-73 - 1700 - Dual carb, manual trans
CD - 73 - 1700 - Dual carb, auto trans
AW - 73-74 - 1800 - Dual carb, manual or auto
AW - 75 - 1800 - Fuel injected
ED - 75 - 2000
GD - 76-79 - 2000 - 79 had Vanagon heads
GE - 79 - 2000 - Cal only, Vanagon heads

Type 3
Code Year Engine Remarks
O - 63-65 - 1500 - No Cam Bearings
TO - 66-67 - 1500 - 6mm oil pump studs
UO - 68-69 - 1600 - Fuel injected ‘69 w/xmember holes
UO - 70 - 1600 - xmember holes, dual relief
U5 - 71-73 - 1600 - like UO w/xmember holes 7.3:1 compresion
X - 72 - 1600 - 7.3:1 compresion

After Market Cases
Code Year Engine Remarks
F1 - 13/1600
F2 - 13/1600
DO - 40-HP part when last digit is "X"
D1 - Some times a 40-HP part
AB - 66-79 - 1600 - Built since ‘73 in Mexico, Brazil and Germany
AK - 67-74 - 15/1600
AM - 1600
AS - sometimes listed as a Super Beetle original case ‘73 and on

Type 4
Code Year Engine Remarks
W - 71 - 1700
EA - 72-74 - 1700
EB - 73 - 1700 - Calif only
EC - 74 - 1700

_________________
When in doubt, GAS IT! It may not solve the problem but it will end the suspense.