from:
http://www.hkpro.com/symbols.htm
"The symbol to the right of the date code is a stag horn. It is the symbol of a particular proof house, located in Ulm, Germany.
German firearms are sent to proof houses before sale, for inspection and quality control. This is much like a 'UL' listing for American small appliances. There are other proof houses, for example, in Kiel and Hannover, Germany, but HK uses the Ulm proof house exclusively."
Proofing helps assure quality, by mandating that someone other than the manufacturer inspect the item. Thats my guess anyway.