I picked up a S&W Model 10-7 Yesterday and have been trying to date it.
SN is 4D502XX
Your 10-7 (and any other Smith with an SN starting with 4D) is from 1979. The 10-7 was introduced in 1977 and succeeded by the the 10-9 in 1988. What about the 10-8, you ask? In the case of the Model 10, S&W used certain "dash" numbers for the tapered barrel versions, which yours is, and different numbers for the heavy barrels. So, the tapered barrel 10-4 was replaced in sequence by the 10-5, 10-7, 10-9 and 10-11. The heavy barrel 10-3 similarly spawned the 10-6, 10-8, 10-10, and 10-12.