No, letter won't help, you're right.
Non-relieved targets are B) on my list below:
As always with things S&W, you will see a lot of overlap in the list below. Serial number ranges would have to be interpolated by model from the years given:
TARGET STOCK STYLE VARIATIONS – ESTIMATED TIME FRAME RANGES:
A) Early non-relieved smooth Rosewood and other wood "Coke Bottle" stocks. Black washer, early 50's, special-order-only-applications, scarce. The bottom flare and long trigger guard area are indicative. Pg 23 SCSW, 3rd ed. The SCSW mentions in the history section for stocks, that Cokes were introduced for both K and N frames in the mid-50's. They were made smooth and checkered, and the K frame checkered are scarce.
B) Non-Relieved checkered Targets. Black washer, 1952 until about 1958.
C) Relieved ("football cutout") checkered and smooth Targets w/diamonds. Black washer, 1956 until about 1974 although most were used up by 1972.
D) Relieved (N "Coke bottle") Target" stocks with Black washer were specific to .44 Magnum thru the 29-2 models and early years of the Model 57. Special order only on pre 24s, 27s, etc., 1956 until 1967-68.
E) Smooth (presentation) Goncalo Alves, relieved. Black washers, used 1967 to 1980.
F) Relieved ("football") checkered w/o diamond, Targets. Silver washer. 1970 until
about 1982.
G) Relieved ("football presentation") Targets. Silver
washer. 1979 until about 1981. These have the "football"
shaped cartridge relief on the left side and the engraving
cut out on the right side and lack checkering.
H) "Speed Loader" Targets. Silver washer. 1980 until about
1991, and infrequently thereafter. Often on L & K frames.
These are cut out for enhanced clearance on
the left side for a speedloader, the scallop exposes
the revolver frame. Late in this issue, the medallion
washer was deleted and the medallions are more or less
force fit into a hole bored in the stocks and the 'alignment
circle' is milled into the wood.
I) "Speed Loader" Presentation Targets. Silver washer. 1979
until about 1991, and infrequently thereafter. These are
cut out for an enhance clearance on the left side for a
speedloader, the scallop exposes the revolver frame.
There is an engraving cut out on the right side. Common
on commemoratives, limited run guns and presentation
pieces. They lack checkering.