In addition to the polarized pin pair for the speaker, the IIfx's MLB has three 2-pin headers that are presumably for selection/connection into circuit of summat-or-other.
J106 and J103 are at grid Q6. On the board that I have, J103 only is jumpered. J20 at N17 is also not jumpered. No combination of IIfx, header, pins, jumpers or baisez mon cul in Google has netted me any info. about the purposes of the header blocks. Can anyone oblige, please?

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I don't know what they're for either, but I did find that the machine wouldn't boot with one jumper configuration and a particular memory installation I was using. However, I didn't take notes and I didn't experiment further. I was too busy getting my SIMMs to work.

I just noted it down as a project for future exploration.

Based on that tiny snippet of info, I imagine that at least one of the jumpers controls something to do with the RAM.