Here's what I found, hopefully it will answer some of your questions.
With a straight grip stock your gun would be a Mountie. They aren't marked with that designation, it's just known that way if it's straight gripped.
In 1954 there were very few Ballard rifled 39A's built! There were only 271 39A's and only 58 39AM (Ballard rifled Mounties) built that year. There were 3,814 39A's with MG rifling, and 1,162 39AM's with MG rifling.
If your gun is straight gripped, and doesn't have the Micro-Groove marked barrel, it's about as rare as a 39A can get!
(here is where I found the above info. that might be useful to you)
http://www.marlinowners.com/forum/marlin-collectors/33052-1954-39a.html
(here is some more discussion)
http://www.marlinowners.com/forum/rimfires/7109-marlin-39a-mountie.html
Marlin year of manufacture maybe determined from the following list of letter/numeral prefixs to the serial number:
1946-C
1947-D
1948-E
1949-F
1950-G
1951-H
1952-J
1953-K
1954-L
1955-M
1956-N
1957-P
1958-R
1959-S
1960-T
1961-U
1962-V
1963-W
1964-Y,-Z
1965-AA
1966-AB
1967-AC
1968-AD, -68
1969-69
1970-70
1971-71
1972-72
Starting in 1973, the year of manufacture maybe determined by subtracting the first two digits of the serial number from 100:
Example: SN 2512345 would have been made in 1975 [100 - 25 = 75].