Our '90 square body Burb had power mirrors similar to the 400 style(like the '99 Burbs and Rawhide have, the black plastic housing that manually folds in). I think the NBS power mirrors are the first factory ones to have a power folding option, our '06 has those.
Yep, I remember those, too. The last few years of R/V Suburbans had the black plastic folding mirrors with power functions, but attached to a base that mounts it to the door just below the window line, IIRC. Since there's no spot for it at the front/A-pillar angle like a gmt400, due to the vent window.
And yes, didn't mean to confuse the subject, all styles of the gmt400 mirrors (and older) would be manual fold as far as I've seen, with gmt800 being when power fold was introduced. The highest option level of the gmt800 mirrors are power, power fold, "dip" on reverse, dimming, heated, LED turn signal in the glass, puddle lamps, and memory. Our SS's have those. They're over $500/ea new, if you can get them.
(I'm sure the "dip" function and memory aren't native to the mirror assembly itself, but are handled by the bcm)
Richard