There's a bit of Nintendo lore that says that certain Nintendo Entertainment System game paks manufactured in 1985 and early 1986 contain that holy grail of adapters, the Famicom to NES converter. For you see although the NES and the Famicom share the same hardware principles, the different shape of their respective game paks and internal circuit boards makes it impossible to simply plug a Famicom game into your beloved NES. However, at the start of the NES generation Nintendo decided against manufacturing unique NES game circuit boards and instead just slapped Famicom boards and the Famicom to NES converter into your standard issue game pak.
Now, all these years later, retro gaming fans have discovered that it's possible to pop the converter out of these early game paks and use it to play Famicom titles on NES hardware. The problem with doing so is that game paks containing the converter aren't exactly labeled as such and later manufacturing runs of the games that did include the converter do not include that blessed piece of hardware.
So how does one find game paks that contain these converters? Redwolf over at Vintage Computing has a few interesting observations regarding the old NES title Gyromite and how it's possible to detect which old game paks have the converter inside without having to pry open the game pak right in the middle of your local used game store.
If you happen to have a scale that can read in ounces, you’ll find that a regular Gyromite cartridge weights about 3.5 ounces, and a FA Gyromite weighs about 4.5 ounces. The extra ounce comes from the converter hardware inside, which you’ll see in a minute. I tested my other early NES games and they all weighed 3.5 ounces, telling me they don’t have adapters inside.
Interesting stuff! Gyromite's finally good for something after all these years.