1986 Alpina

I am stationed in germany and found an Alpina for sale and jumped on it, but i dont know if it just has Alpina parts on it or it is an actual Alpina. has a straight 6 in it and is fast as lightning ( raced many others on the autobaun ). All i can find under the hood is a metal plate with M5 on it ( 535i motor). Is there anyway to determine if it is the real thing?

Thanks in advance, Luke in Germany