ww2 you sure can pack a lot of questions into a few sentences!
OK, the only difference between a RC officers and a Soc. Welfare dagger are the type of holes used to connect the hanger (Soc. Welfare = Circular, and RC officers = square).
The thing to remember is that these differences have never (to my knowledge) been proven to truly represent a differnece in thedesignation of the dagger. It may VERY well be that the difference in the holes has to do with the manufacturer of the dagger, not what branch of the RC it was for.
Now these daggers have been found usually with the standard Heer portapees (as Dion correctly mentioned). Veyr rarely you can find them equiped the the very similar 42cm aluminum portapee with the "red dot".
Some people say that the red dot is correct for RC officers and that the all aluminum port is correct for social "welfare officer" daggers. But again I think most of this is based upon collectors opinons and very little in fact.
It I were you I would put the standard aluminum portapee on your dagger. If you really want a red-dot one you can get a nice repro from Tom Johnson (ww2daggers.com) but finding an original re-dot port would take a long time (and $$$).
And finally, yes there is a specific way to tie a portapee on a RC officers/ soc. welfare dagger. It is kind of a tricky knot. I think that in one of Johnsons books there is an illustration of how to tie it. I will look around for you and if I find it I will post a pic.
Good luck and good hunting!
Johnny