I'm making a final entry here, in case it might be of use to anyone in the future.
I've completed my installation, differently from what is illustrated in Realoem and the other parts catalogs based on the online versions of the BMW white books--also differently from what a number of other users of this forum proposed.
The consensus in previous postings and those who responded to my posting was, from outside in, (1) nut, (2) thin flat washer, (3) arm, (4) thicker flat washer, chassis. Straightforward: The arm is surrounded by washers and secured with a nut.
But the "part manuals" differ, showing from outside in, (1) thin M8 nut, (2) split stainless-steel (!) M8 spring washer, (3) M8 flat washer, arm, chassis. Two of these are special parts: the thin nut and the spring washer. BMW must have had a reason to use them, though I'm not sure what it was. The strange thing in this setup is that there's no washer to isolate the arm from friction with the chassis.
I decided to use the three BMW-specified BMW parts along with an extra washer on the inside:
I ponied up for the BMW parts only to find that the washers were nothing special and I could have had them from any hardware store with metric stock. The thin nut, however, provides a finished look if an extra washer is inserted against the chassis. Otherwise, a regular nut would just barely thread completely onto the shaft.
Thanks again to the kind, helpful, and patient fellow users of this forum who responded to my post.