I'm not sure it's all neglect....My first K98k was one such 'byf44' in the l block range....The receiver and barrel were blued, while the butt plate, entire bolt, trigger guard and bayonet lug all had a phosphate finish....Was missing the takedown disk and had the hole in the BP....
After having the rifle for a while, I noticed the phosphate parts were taking on a brownish appearance, to which I figured I had left those parts unoiled....I continued using a petroleum-based oil and the condition worsened...I eventually sold it back to the guy I bought it from, and he figured I shouldn't have used ordinary gun oil on it....Since then, I resist using any such oil on any German phosphate finishes on my rifles/pistols and haven't yet reexperienced such problems....Bodes