i have 92 gs500 which i believe engine has been change reason i need to know year is i need to replace head if anyone can help me here is engine code m504-113412 ??

What color is the engine?
All heads are compatible except in 2004+ they dropped the tachometer gear and went electronic. You can put on an electronic tach if you can't find a 1989-2002 head.
So year doesn't matter except for the tachometer and color.

ok and where would that be located at color of engine is silver

Maybe this would help:

http://www.suzukicycles.org/GS-series/GS500E.shtml

There was a thread... um, couple years ago now I think. Quite a few of us listed our engine numbers and year as a kind of rough reference. Cannot find it now 

Anyway. For reference. My 09 has an engine number of M504-153619.

If some others with older than 09 can put up their engine numbers we should be able to get close to your number.

The thread MR referenced: http://gstwins.com/gsboard/index.php?topic=57564.msg652258#msg652258

Looks like your engine has been replaced, as GS500s before '01 had M501 engines. To determine whether it is '04+, look at the front near the top. If a cable is attached, it is a mechanical tach, and pre-2004.