TINK nailed it, above ^
The "key clue" is the notch for the perch-tightener bolt. The little wedge bugger is a thing of beauty: the underside (what doesn't show here) is curved to fit the bars, and has sharp edges so that, when clamped down, it grips the bar.... Old School Engineering
I learned about it on my Slash 2, then had the same on my Slash 6, and MUCH to my surprise, my "new" R80ST has one!!!
...but as others have found out, if it's missing, it ain't critical for function, and especially since MAX BMW shows it as NLA.