Manufactured in 1973. 4K numbers were used in three different years, 1972, 1973 and 1974. The range for 1973 is 4K1628 to 4K54104. As muddocktor stated, it could have shipped in 1974, since S&W did not ship guns in serial order.
Your Combat Magnum is in very nice condition, although the stocks are later than the gun. I'd put the current price in the $500 range.