To the best of my knowledge, these numbers were first applied on June 21,or June24, 1948. I believe that the numbers represented the different districts. Mass socks of the "Sower" issue were overprinted by the Allied forces after the collapse of Nazi Germany. Maybe someone with more expertise in this area can help?
These are postage stamps issued in the Russian zone as an emergency measure following a currency reform.
There are thousands of these overprints which follow a common format of "OPD" number plus the postal district name
The OPD (Oberpostdirektion) number identifies the old high-level German postal administration regions. Here "16" refers to OPD Erfurt in Thuringia.
My 1999 Michel Germany Specialised catalogue lists all the postal districts that were involved in this overprinting within each OPD.
Here the postal district is Sitzendorf.
Many of these overprints were forged in large quantities and the most common districts to be forged are identified in Michel with a zero price.
I'm pleased to say Sitzendorf is not one of these.
I assume the postmark of Mellenbach is the current village of Mellenbach-Glasbach which is very close to Sitzendorf.
My Michel catalogue gives prices for each postal district and for each basic stamp overprinted like this within each OPD. However, it does not go as far as listing each stamp known from each postal district!
I don't know if this information is still published in the Michel Germany Specialised. I believe it may have been removed.
I hope this helps along with the article provided by Jlav.