The US made HK45's are HKU-XXXXXX. The US made HK45C's are 216-XXXXXX. The German HK45C is 128-XXXXXX. I don't know what the German prefix is for the HK45, however.
I think the HKU thing was for all US HK's originally, but went against the prefix identifying SN's that HK was previously using. That's an educated guess though.