I'll take one of mine out and take pictures and measure it. Not sure if it will be tonight though.
I'll also see if I can find another card that has the same bracket positioning as I have a massive number of video cards.
Edit: That was quicker than expected.
Found 6 different cards that have brackets that will work.
Click pics for full size.
EXACT matches including the bracket mounting holes on the cards:
From top to bottom:
ATI Mach64
ATI Rage 128 full height - probably the easiest and cheapest card to get
ATI RAGE II + DVD
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Matches from top to bottom:
ATI Rage XL - probably also super easy and cheap to get - you would need to cut the mounting tab off of the bracket though.
ExpertColor branded S3 Trio/Virge - no mounting tabs
Matrox Millennium 2 - No mounting tab and you would have the hole from the MAC style 15-pin connector - going to be the most flimsy
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Voodoo 3 3000 PCI:
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Voodoo 3 3000 PCI bracket measurement:
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