SLC Glass is evident by the curved top edge. I believe the A-pillar line is actually the same, but the top part is curved on an SLC (I believe).

I am not familiar with the 5 mounting points issue. There is a variation in the type of top and bottom stops over the years. So I wonder if the middle tab you are talking about is actually a stop mount tab. I look forward to the pictures.

Are your windows manual or power?

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Past mb: '73 450sl, '81 280slc stick, '71 250, '72 250c, '70 250c, '79 280sl, '73 450sl, parted: '75 240d stick, '69 280s, '73 450slc, '72 450sl,