The 3 first numbers/letters and the other numbers (the number of making) give the model code of the bike.
Like as you can see below, the 31K, 31W, 1JF, 1JG, 57V and 1AF have the  engine/frame number beginning by 31K; this is the number of making who make the difference (The beginning of engine/frame number is not obviously the model code of the bike). The model code can change only for a new coloring ('86 1WT and '88 3DH1 for example).
The model code is writted on the microfiches like on this one of '86 RD350N2 model code 1UA.

Be careful when you buy a second-hand 350LC: check that the serial number is the same on the engine and on the frame (and on papers of course) ! if they are different, it means that the engine was broken and replaced for example. Be careful !: the numbers should match but the engine # can be re-stamped.

Genuine engine # stamped by YAMAHA

Re-stamped # (on smooth surface to erase the geniune #)