You may have a better and more complete answer already from the S&W forum. According to the Standard Catalog of Smith and Wesson 4th Edition, K Frame revolver serial number range for 1948 is K18732 - K73121. So yours is an early gun and desirable. Value depends on several factors with condition being prime. Value is enhanced if you have the original box, papers, manual, tools, etc. Catalog says excellent condition valued at $1100, very good $800.