The pre 1936 years used A, C and a few other letters. Parley Baer has a very good handle on those years. I'm still confused in that area, but he's worked on it several years at least to get to the bottom of it.
Here's the notes I have on the Models 1936 and 36. Most of this info came from a Marlin collector who specializes on the 1936 and 36:
Model 1936: 1936 to 1941.
30-30, 32 Special
The 1936 had case colored receivers & Model 1936 on upper tang.
1936 1st series production (no letter prefix serial numbers) had flat mainspring, fluted comb buttstock, perch belly forearm, hard rubber buttplate w/slotted screws. Starting in 1937, this model was listed in Marlin's Catalogs as the Model 36 and was only there referred to as the 36. The gun itself was still stamped Model 1936 until sporting arms production was discontinued in 1941. Serial numbers located on the lower tang under the lever.
Regular carbine 20" barrel and full magazine. Sporting carbine 20" barrel and half magazine. Rifle 24" barrel and half magazine. Deluxe included checkering and installed sling.
B-prefix series: Changes to unfluted comb & beavertail forearm, hard rubber buttplate w/ phillips screws, upper tang was shortened and the use of coil mainsprings vice leaf mainsprings. 1941 Production.
No Letter Prefix-1936 to 1940
No WWII Production Late 1941 to late 1945
Model 36: 1945 to 1947.
30-30, 32 Special
Model 36 is marked on the barrel, not on the tang as in the Model 1936 and earlier guns. Blued reciever. Serial numbers located on the lower tang under the lever. These are squarebolts and are not to be confused with later roundbolt Glenfield 36 (336 roundbolt action). Late 1945 production resumed on limited basis using C prefix and leftover parts (some guns will have Pre-WWII Model 1936 parts mixed with Post WWII Model 36 parts. IE: 1936 action with 36 barrel. These guns are considered Model 36s.
Post WWII production had blued receivers and Model 36 on barrel
Barrel roll stamp reads:
"The Marlin Firearms Co. New Haven Conn USA-est 1870"
Model 36 (then caliber designation)
Variations of Model #: 36-A-DL, 36-A, 36-RC
C-1945 shiny blue on top of receiver
c-1946 (note: c prefix is lower case)
D-1947 sandblasted top of receiver starts
Per the Marlin Collector's Association, the Model 36 production ceased in 1948. And the Model 336 production began in June 1948. Which would mean to me that there is a probability that there could be some E prefix Model 36s out there somewhere.