First I scrounged around for some Addi needles, and luckily found some Turbo straights, clearly marked as US 8 / 5mm. Then I grabbed a handful of Ringlets (because you know I have handfuls all over the place), and set to work. And lo. Some Ringlets fit on the 5mm Addi's, and some didn't. Weird. So I pulled out the needle gauge nearest to hand (sold by Lion Brand Yarn) and found that the Addi's didn't fit the 5mm hole -- it wasn't that they were just tight, they did not fit through the hole at all. Okay, weird.
More testing ensued. I pulled out a variety of 5mm needles, choosing different materials and manufacturers. Besides the Addi's, some vintage Boye plastic needles were the only ones that did not fit the 5mm hole in the Lion needle gauge. Lantern Moon wood needles and Clover bamboo needles fit, albeit very tightly, while Hiya Hiya and Clover Takumi bamboo and Boye aluminum needles fit just fine. Okey dokey.
Then I pulled out two other needle gauges (the only ones I could find). One was a plastic gauge with The Fold's imprint on it, and the other was a laser-cut wooden gauge given as a handout by a LYS. The Addi's just barely fit into the 5mm hole on these gauges. All of the other 5mm needles fit fine except for the Lantern Moon wood needles, which barely fit in the LYS wooden gauge.
So there you have it. Addi's 5mm is bigger than other manufacturer's 5mm. Not all 5mm needles are created equal. And neither are needle gauges.
Ringlets. They fit up to 5mm needles, unless your
5mm needles are not the same size as my 5mm needles.