I haven't grabbed a Rev A/B, but I have stored pictures away as the auctions have gone by. Here's what I've seen:
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First, there's the Griffin iPort, which uses the physical slot opening but not actually the mezzanine interface. The PCB runs cables to the standard 8-pin mini-din serial connector for the infrared port and the DB15 port for the internal CRT to provide both interfaces externally.
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Then, there's the iPro RAID, which was delivered in two versions; one with an ultrawide SCSI controller and another which also bundled analogue video (/tuner?) input. (only one picture of this available) This card does use the mezzanine interface. I actually have one of the SCSI-only models in box, despite not yet finding an iMac to put it in...
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And finally (that I know of) the 3dfx Game Wizard, which adds Voodoo II acceleration to the onboard display.
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And also, the Sonnet Harmoni G3 500 or 600MHz, which occupies neither the Mezzanine interface nor the slot opening when installed.
Alas, all are too expensive for me (I thought $400 for one back in 2005 was expensive, but in retrospect, that price is actually quite affordable, because these often go for 2x more!).
i also have an iMac G3 Rev. B with the Sonnet G3/600 Harmoni in it, got that Sonnet Card for 25 Euros, it was in a broken iMac (seller didn´t know what he had...)