Marlin's system is screwy and inconsistent, but I strongly believe the following to be correct:
1894C - Currently (and since 2001) a 18 1/2" 357 mag. From 1979 - 1984, the 1894C was the same as today (inlc. .357 mag) except it did not have the hammer block safety.
1894CS - What the current 1894C .357 mag was called between 1984 and 2001. "S" denotes the addition of the safety in 1984.
1894CP - 16 1/4" ported bbl in .357 mag. Discont'd.
1894P = 16 1/4 ported .44 mag. discont'd.
1894SS - Current stainless version of 1894 in .44 mag.
1894 - Currently manuf .44 mag w/a 20" bbl. This model was called the 1894 from 1969 - 1984, and then the 1894S from 1984 until 2001 (just like the 1894C/CS), again the "S" denoting the added hammer block safetybeginning in 1984. It was changed back to 1894 in 2001.
The 1894 w/out any special designation was also manuf in .41 mag and .45 Colt at various times. The 1894 in .41 mag is now catalogued as the 1894FG.
there are some other discon't variants:
1894S Limited in a 16 1/4" bbl manuf in 1996.
1894SC LTD in .44 mag (1996) or .45 Colt (1997), both w/16 1/4" bbl.
1894CL in .218 Bee (highly prized) and some cowboy calibers in late 80's and early 90's.
plus a few others.
The "S" became superfluous when all of the Marlins got the safety, but it took them a while to change back. Numbering system of the 336 and 1895 has been similarly tinkered with.
I would trade my left one for a 1894CL in .218 Bee if anyone knows of one.