Whichever parts they are, the meat is recovered mechanically and, in common with most pink coloured, mushed meat products, is the real dreg ends of the dreg ends that you'd never want to eat in a million years, even if you were so hungry that you were within a whisker of gnawing off your own arm and eating it raw.
The fact that the texture of luncheon meat is slightly gritty when you bite it ought to tell you it has bits of bone in. Eeeeew.
if you buy the 'better' sausages its not the purreed pink stuff because you can make out the 'sausage meat' so to speak. does that make any sense?
A bit like buying really cheap beefburgers and good ones just made of beef or venison meat.