I think I've figured out the date codes on European Osram Sylvania general purpose lamps.

On some of my French-made Sylvania A19 halogens, I have the date code "h418"  The letter represents the factory location, the first number represents the year of manufacture, the second number/letter represents the month of manufacture, and the fourth number represents the type of date code used.

For example, h418 represents January 2014

The months are as follows:

1-January
2-February
3-March
4-April
5-May
6-June
7-July
8-August
9-September
A-October
B-November
C-December

h598-September 2015
h5A8-October 2015
etc.