I have one of the "plum" slide pistols and its story was that they were German law enforcement issues that went back to HK for refurb. The bluing was flawed and resulted in the plum coloring. Mine also has a milled blemish on the slide that I was told had the unit marking of the police agency to which it had been issued. All this is second hand.

I will agree that they are amazing bits of engineering and a joy to shoot.

Values are a mystery to me other than that they seem to continually go up.