Hoping someone here knows the answer. I have an S&W Model 15-6, apparently stainless steel, with a special feature code on the box of 2118. Does anyone know what that code would indicate?

Nice gun, which I bought used. Red insert front sight with white outline on the rear sight blade. Some Celtic style line engraving on each cylinder flute and on each end of the roll marks, plus several places on the frame. Deep texture stag grips. Great shooter.

Thanks for the help.