I am a licensed US Customs Broker and can safely state that there are more than one way to mark a product manufactured in a foreign country. Among other options, a tag or sticker affixed to the product removable only by the original purchaser usually makes a product legally marked. If a Yamaha trumpet simply says "Japan" anywhere, even on its bell, it would very likely be considered legally marked. (US laws state markings must be placed in a conspicuous manner). CBP does have some exceptions on products about their lack of country of origin markings, but these exceptions are too detailed to put here.
I believe Kanstul once had a bit of a problem with their USA made versions of the French Besson as they copied the engraving including "Made in France". That is an illegal marking both in the US and France.
For the record, although a vast majority are marked as such, USA made products do NOT have to be marked for distribution in the US. US products sent to a foreign country will have to be marked in such a manner as to meet a particular country's requirements.