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The Serial Number and pertinent FCC Regulation information for our particular IBM PC Jr. We don't have a modem in ours so the second regulation does not apply.
One of the interesting and unique features of the PC Jr. was the side panel expansion system.
Here we are looking at the expansion panel for a parallel printer port. My personal PC Jr., all those years ago, had a 128KB RAM expansion board. (That's right, a whopping 256KB of RAM total)
The plastic door of the expansion pops right off to reveal several screws and another port of an additional expansion if you so wished.