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Stickers shouldn't mean anything to anyone needing to know whether a vehicle complies with a state's requirements. It will be on the emissions "approved list". I've never looked for stickers on mine and would expect an inspector to know exactly where they are.
You are correct; in theory. I've purchased many out of state motorcycles over the years and titled them in California. The inspector in every case looked for two compliance labels affixed to the motorcycle in question. Federal Safety Compliance label (which has the vehicle's VIN# printed on the label) and the Vehicle Emissions Compliance Label. No label, no pass. That is of course in addition to the VIN# embossed on the right side of the headstock and the motorcycle's engine #. This last one, depending the the bike and where the # is embossed, has required me in some cases to remove quite a bit of stuff to allow it to be visible. And in one case where the two mandatory stickers were removed by the previous owner, I paid to have a local sign shop make some new stickers.
Last year I registered/titled 2 out of state motorcycles, both Rokons, and when I went to the 3rd party MVD place, they weren't even interested in seeing the vehicles, much less looking for any emissions stickers or V.I.N.s. All I needed was the out of state titles. I guess the rules have changed in the last 23 years. In AZ now you don't even need to get a title notarized, the registered owner just signs the title when the vehicle is sold.
I love the quickie-registration,3rd-party, place near my place north of Phoenix (my winter golf/ pre-season baseball place). It's never busy and they are fast and polite.