I just looked it up on Partstree.com and it shows the 5247 being produced from 1978 through 1981, it was dropped for 1982, then produced again in 83 and 84. The first letter, or number of the serial number will determine the year that it was produced. Just be aware this system is flawed when it comes to determining year of production on many of these Lawnboy models. I have seen plenty of instances where the serial would indicate a year, but the Toro database doesn't even show that model produced that year, so we can't really nail down an exact year on those models.
1978- 8XXXXXXXX
1979- 9XXXXXXXX
1980- 0XXXXXXXX
1981- 1XXXXXXXX
1983- BXXXXXXXX
1984- CXXXXXXXX